Memory system and semiconductor memory device

ABSTRACT

According to one embodiment, a memory system includes a semiconductor memory device and a controller. The semiconductor memory device includes a first memory cell configured to store data. The controller is configured to output a first parameter and a first command. The first parameter relates to an erase voltage for a first erase operation with respect to the first memory cell. The first command instructs the first erase operation. The controller outputs the first command after outputting the first parameter to the semiconductor memory device.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromthe Japanese Patent Application No. 2020-124259, filed Jul. 21, 2020,the entire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a memory system and asemiconductor memory device.

BACKGROUND

As a non-volatile semiconductor memory device, for example, a NAND flashmemory, in which memory cells are two-dimensionally orthree-dimensionally arranged, is known. A NAND flash memory and acontroller that controls the NAND flash memory form a memory system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a configuration of a memory systemaccording to a first embodiment.

FIG. 2 is a block diagram showing a configuration of a memory chip in asemiconductor memory device.

FIG. 3 is a circuit diagram of a block in a memory cell array.

FIG. 4 is a cross-sectional view of a partial region of a block with thememory cell array.

FIG. 5A is a diagram showing a relationship between data and thresholdvoltage distributions that may be observed in memory cell transistors.

FIG. 5B is a diagram showing the degree of erasure in memory celltransistors after an erase operation.

FIG. 6 is a diagram showing a basic erase operation in the memory systemaccording to the first embodiment.

FIG. 7 is a diagram showing voltage waveforms of respective signals inthe erase operation.

FIG. 8 is a flowchart showing a first example of the erase operation inthe memory system according to the first embodiment.

FIG. 9 is a diagram showing operations performed between a memorycontroller and the semiconductor memory device.

FIG. 10 is a diagram showing an example of a pulse time management tableprovided in a memory of the memory controller.

FIG. 11A is a flowchart showing processing of “determining a pulse timeof an erase voltage VERA” in FIG. 8.

FIG. 11B is a diagram showing a relationship between a reference valueY1 in FIG. 11A and the number of times a write operation/erase operationis performed.

FIG. 12 is a diagram showing a threshold voltage distribution and ajudgment level, which indicate a method of determining the degree oferasure in the first example of the erase operation.

FIG. 13 is a diagram showing an example of the number of OFF bits storedin a buffer when a read is performed at a judgment level.

FIG. 14 is a diagram showing another example of the number of OFF bitsstored in the buffer when the read is performed at the judgment level.

FIG. 15 is a diagram showing a command sequence in the first example ofthe erase operation according to the first embodiment.

FIG. 16 is a flowchart showing processing of “determining a pulse timeof an erase voltage VERA” in a modification of the first example.

FIG. 17 is a flowchart showing a second example of the erase operationin the memory system according to the first embodiment.

FIG. 18 is a flowchart showing processing of “determining a pulse timeof an erase voltage VERA” in FIG. 17.

FIG. 19 is a diagram showing threshold voltage distributions andjudgment levels, which indicate a method of determining the degree oferasure in the second example of the erase operation, and data used forcalculating the number of OFF bits.

FIG. 20 is a diagram showing a command sequence in the second example ofthe erase operation according to the first embodiment.

FIG. 21 is a diagram showing a relationship between the number of timesa write operation/erase operation as a comparative example is performedand the degree of erasure in memory cells by the erase operation.

FIG. 22 is a diagram showing a relationship between the number of timesa write operation/erase operation according to the first embodiment isperformed and the degree of erasure in memory cells by the eraseoperation.

FIG. 23 is a flowchart showing a first example of the erase operation inthe memory system according to a second embodiment.

FIG. 24 is a diagram showing operations performed between a memorycontroller and the semiconductor memory device.

FIG. 25 is a diagram showing an example of a voltage value managementtable provided in a memory of the memory controller.

FIG. 26 is a flowchart showing processing of “determining an initialvoltage value of the erase voltage VERA” in FIG. 23.

FIG. 27 is a diagram showing a command sequence in the first example ofthe erase operation according to the second embodiment.

FIG. 28 is a flowchart showing a second example of the erase operationin the memory system according to the second embodiment.

FIG. 29 is a flowchart showing processing of “determining an initialvoltage value of the erase voltage VERA” in FIG. 28.

FIG. 30 is a diagram showing a command sequence in the second example ofthe erase operation according to the second embodiment.

FIG. 31 is a flowchart showing a first example of an erase operation ina memory system according to a third embodiment.

FIG. 32 is a diagram showing operations performed between the memorycontroller and the semiconductor memory device.

FIG. 33 is a diagram showing an example of a management table for apulse time and a voltage value, provided in a memory of the memorycontroller.

FIG. 34 is a diagram showing a command sequence in the first example ofthe erase operation according to the third embodiment.

FIG. 35 is a flowchart showing a second example of the erase operationin the memory system according to the third embodiment.

FIG. 36 is a diagram showing a command sequence in the second example ofthe erase operation according to the third embodiment.

FIG. 37 is a diagram showing a command sequence in a first example of anerase operation according to a fourth embodiment.

FIG. 38 is a flowchart showing a second example of the erase operationin a memory system according to the fourth embodiment.

FIG. 39 is a flowchart showing processing of “determining a pulse timeof the erase voltage VERA” in FIG. 38.

FIG. 40 is a diagram showing threshold voltage distributions of memorycells corresponding to judgment levels AR1 to AR4 used in the processingshown in FIG. 39.

FIG. 41 is a diagram showing a relationship between the number of OFFbits obtained at the judgment levels AR1 to AR4 and erasure states.

FIG. 42 is a flowchart showing processing of “determining a pulse timeof the erase voltage VERA” in a third example of the erase operationaccording to the fourth embodiment.

DETAILED DESCRIPTION

In general, according to one embodiment, a memory system includes asemiconductor memory device and a controller. The semiconductor memorydevice includes a first memory cell configured to store data. Thecontroller is configured to output a first parameter and a firstcommand. The first parameter relates to an erase voltage for a firsterase operation with respect to the first memory cell. The first commandinstructs the first erase operation. The controller outputs the firstcommand after outputting the first parameter to the semiconductor memorydevice.

Hereinafter, the embodiments will be described with reference to thedrawings. In the following description, constituent elements having thesame function and configuration will be assigned common referencenumerals. The embodiments to be described below are shown as an exampleof a device or a method for embodying the technical idea of theembodiments, and are not intended to limit the material, shape,structure, arrangement, etc. of components to those described below.

Each of the function blocks can be implemented in the form of hardware,computer software, or a combination thereof. The function blocks are notnecessarily categorized as in the later example. For example, part ofthe functions may be implemented by a function block other than theexemplary function blocks. In addition, the exemplary function blocksmay be further divided into functional sub-blocks. Hereinafter, athree-dimensionally stacked type NAND flash memory in which memory celltransistors are stacked above a semiconductor substrate will bedescribed as an example of the semiconductor memory device included inthe memory system.

1. First Embodiment

A memory system according to a first embodiment will be described.

1.1 Configuration

1.1.1 Configuration of Memory System

First, a configuration of a memory system according to the firstembodiment will be described with reference to FIG. 1. FIG. 1 is a blockdiagram showing a configuration of the memory system according to thefirst embodiment. A memory system 1 includes a semiconductor memorydevice 10, a memory controller 20, and a buffer memory 30. The memorysystem 1 is coupled to an external host device 2 and is configured toexecute various operations in response to instructions from the hostdevice 2.

The semiconductor memory device 10 includes one or more memory chips10_0, 10_1, 10_2, . . . , 10_n (n is a natural number equal to orgreater than 0). The memory chip 10 n includes a plurality of memorycells and stores data in a non-volatile manner. The semiconductor memorydevice 10 will be described later in detail.

The memory controller 20 is coupled to the semiconductor memory device10 via a NAND bus. The NAND bus receives and transmits signalscompatible with a NAND interface. The memory controller 20 is coupled tothe host device 2 via a host bus. The memory controller 20 controls thesemiconductor memory device 10. The memory controller 20 accesses thesemiconductor memory device 10 in response to an instruction receivedfrom the host device 2.

The buffer memory 30 temporarily stores write data and read datatransmitted and received between the semiconductor memory device 10 andthe host device 2. The buffer memory 30 is comprised of, for example, adynamic random access memory (DRAM), a static random access memory(SRAM), etc.

The semiconductor memory device 10 and the memory controller 20 may formone semiconductor device in combination, for example. Examples of such asemiconductor device include a memory card including an SD™ card, asolid state drive (SSD), etc. The memory controller 20 may be asystem-on-a-chip (SoC), for example.

The host device 2 is, for example, a digital camera, a personalcomputer, etc., and the host bus is, for example, an SD™interface-compatible bus.

1.1.2 Configuration of Memory Controller 20

A configuration of the memory controller 20 will be described withreference to FIG. 1 again. The controller 20 includes a centralprocessing unit (CPU) (or processor) 21, a memory 22, a host interface(host I/F) 23, an error checking and correcting (ECC) circuit 24, a NANDinterface (NAND I/F) 25, and a RAM interface (RAM I/F) 26.

The CPU 21 controls the operation of the memory controller 20. Forexample, upon receipt of a write instruction from the host device 2, theCPU 21 issues, in response thereto, a write instruction to the NANDinterface 25. Similar processing is performed when reading and erasingdata. The CPU 21 executes various types of processing such as wearleveling, for managing the semiconductor memory device 10. Operations ofthe memory controller 20, which will be described hereinafter, may berealized by the CPU 21 executing software (or firmware) or may berealized by hardware.

The memory 22 is, for example, a semiconductor memory such as a DRAM oran SRAM, and is used as a work area of the CPU 21. The memory 22 storesa buffer 22A configured to store various types of information, firmwarefor managing the semiconductor memory device 10, various types ofmanagement tables 22B, etc. The buffer 22A stores, for example,information on an erase result of a memory cell after an erase operationor after a write operation subsequent to the erase operation, that is,information indicative of an erasure state in a memory cell group (forexample, a block) which is a target of erasure after the erase operationor the write operation. The management table 22B includes, for example,a parameter of an erase voltage VERA associated with each block. Theparameter includes a pulse time or an initial voltage value of the erasevoltage VERA. The management table 22B includes management tables 22B_1,22B_2, and 22B_3 to be described later.

The host interface 23 is coupled to the host device 2 via a host bus,and controls communications with the host device 2. The host interface23 transfers instructions and data received from the host device 2 tothe CPU 21, the memory 22, and the buffer memory 30, respectively. Inresponse to an instruction from the CPU 21, the host interface 23transfers data in the buffer memory 30 to the host device 2.

The ECC circuit 24 executes error correction processing on data. At thetime of a write operation, the ECC circuit 24 generates parity based onwrite data received from the host device 2, and adds the generatedparity to the write data. At the time of a read operation, the ECCcircuit 24 generates a syndrome based on read data received from thesemiconductor memory device 10, and detects and corrects errors in theread data based on the generated syndrome.

The NAND interface 25 is coupled to the semiconductor memory device 10via a NAND bus, and controls communications with the semiconductormemory device 10. The NAND interface 25 transmits various signals to andreceives various signals from the semiconductor memory device 10 basedon instructions received from the CPU 21.

1.1.3 Configuration of Semiconductor Memory Device 10

Next, a configuration of the semiconductor memory device 10 will bedescribed. As shown in FIG. 1, the semiconductor memory device 10includes the plurality of memory chips 10_n. The memory chip 10_nincludes, for example, a NAND flash memory capable of storing data in anon-volatile manner.

1.1.3.1 Configuration of Memory Chip

A configuration of the memory chip 10_n will be described with referenceto FIG. 2. FIG. 2 is a block diagram illustrating a configuration of thememory chip 10_n in the semiconductor memory device 10. The memory chip10_n includes a memory cell array 11, an input/output circuit 12, alogic control circuit 13, a ready/busy circuit 14, a register group 15,a sequencer (or a control circuit) 16, a voltage generator 17, a driver18, a row decoder module 19, a column decoder 31, and a sense amplifiermodule 32. The register group 15 includes a status register 15A, anaddress register 15B, a command register 15C, and a register 15D.

The memory cell array 11 includes one or more blocks BLK0, BLK1, BLK2, .. . , and BLKm (where m is an integer equal to or greater than 0). Eachof the blocks BLK0 to BLKm includes a plurality of memory celltransistors (hereinafter also referred to as “memory cells”) eachassociated with a row and a column. The memory cell transistors arenonvolatile memory cells capable of being electrically erased andprogrammed. The memory cell array 11 includes a plurality of word lines,a plurality of bit lines, and a source line for applying voltages to thememory cell transistors. Hereinafter, a “block BLKr” (where r is aninteger equal to or greater than 0 and equal to or less than m) refersto each of the blocks BLK0 to BLKm. A specific configuration of theblock BLKr will be described later.

The input/output circuit 12 and the logic controller 13 are coupled tothe memory controller 20 via an input/output terminal (or a NAND bus).The input/output circuit 12 transmits and receives an I/O signal DQ(such as DQ0, DQ1, DQ2, . . . , or DQ7) to and from the memorycontroller 20 via the input/output terminal. The I/O signal DQcommunicates a command, an address, data, etc.

The logic controller 13 receives an external control signal from thememory controller 20 via the input/output terminal (or NAND bus). Theexternal control signal includes, for example, a chip enable signal CEn,a command latch enable signal CLE, an address latch enable signal ALE, awrite enable signal WEn, a read enable signal REn, and a write protectsignal WPn. The symbol “n” assigned to the name of a signal indicatesthat the signal is active-low.

The chip enable signal CEn allows a memory chip 10_n to be selected, andis asserted when this particular memory chip 10_n is selected. Thecommand latch enable signal CLE allows a command transmitted as a signalDQ to be latched in the command register 15C. The address latch enablesignal ALE allows an address transmitted as a signal DQ to be latched inthe address register 152. The write enable signal WEn allows datatransmitted as a signal DQ to be stored in the input/output circuit 12.The read enable signal REn allows data read from the memory cell array11 to be output as a signal DQ. The write protect signal WPn is assertedto prohibit writing and erasure with respect to the memory chip 10_n.

The ready/busy circuit 14 generates a ready/busy signal R/Bn in responseto control by the sequencer 16. The ready/busy signal R/Bn indicateswhether the memory chip 10_n is in a ready state or in a busy state. Theready state indicates that an instruction from the memory controller 20can be received. The busy state indicates that an instruction from thememory controller 20 cannot be received. Through reception of theready/busy signal R/Bn from the memory chip 10_n, the memory controller20 can recognize whether the memory chip 10_n is in the ready state orin the busy state.

The status register 15A stores status information STS required foroperations of the memory chip 10_n, and transfers this statusinformation STS to the input/output circuit 12, based on an instructionfrom the sequencer 16. The address register 152 stores an address ADDtransferred from the input/output circuit 12. The address ADD includes arow address and a column address. The row address includes, for example,a block address that specifies an operation target block BLKr, and apage address that specifies an operation target word line WL in thespecified block. The command register 15C stores a command CMDtransferred from the input/output circuit 12. The command CMD includes,for example, a write command that instructs the sequencer 16 to performa write operation, and a read command that instructs the sequencer 16 toperform a read operation. The register 15D stores a parameter (forexample, a pulse time or an initial voltage value) of the erase voltageVERA transmitted from the memory controller 20 and transferred from theinput/output circuit 12. The register 15D includes registers 15D_1 and15D_2 to be described later. For example, an SRAM is used for the statusregister 15A, the address register 15B, and the command register 15C,and the register 15D.

The sequencer 16 receives a command from the command register 15C, andcollectively controls the memory chip 10_n in accordance with a sequencebased on the received command. The sequencer 16 controls the row decodermodule 19, the column decoder 31, the sense amplifier module 32, thevoltage generator 17, etc., thereby executing a write operation, a readoperation, and an erase operation. Specifically, the sequencer 16controls the row decoder module 19, the driver 18, and the senseamplifier module 32 based on a write command received from the commandregister 15C, and writes data into a plurality of memory celltransistors each of which is designated by the address ADD. Thesequencer 16 controls the row decoder module 19, the driver 18, thecolumn decoder 31, and the sense amplifier module 32 based on a readcommand received from the command register 15C, and reads data from thememory cell transistors each of which is designated by the address ADD.The sequencer 16 further controls the row decoder module 19, the driver18, the column decoder 31, and the sense amplifier module 32 based on anerase command received from the command register 15C, and erases datastored in a block designated by the address ADD.

The voltage generator 17 receives a power supply voltage via a powersupply terminal (not shown) from outside of the memory chip 10_n. Byusing this power supply voltage, the voltage generator 17 generatesvoltages required for a write operation, a read operation, and an eraseoperation. The voltage generator 17 supplies the generated voltages tothe memory cell array 11, the driver 18, and the sense amplifier module32.

The driver 18 receives a plurality of voltages from the voltagegenerator 17. Of the voltages supplied from the voltage generator 17,the driver 18 supplies voltages respectively selected for a readoperation, a write operation, and an erase operation to the row decodermodule 19 via a plurality of signal lines. The driver 18 supplies, forexample, the erase voltage VERA to a well interconnect CPWELL to bedescribed later, at the time of an erase operation.

The row decoder module 19 receives a row address from the addressregister 15B, and decodes the received row address. The row decodermodule 19 selects one of the blocks based on a result of decoding therow address, and selects a word line WL in the selected block BLKr. Therow decoder module 19 transfers voltages supplied from the driver 18 tothe selected block BLKr.

The column decoder 31 receives a column address from the addressregister 15B, and decodes the received column address. The columndecoder 31 selects a bit line based on a result of decoding the columnaddress.

In a read operation of data, the sense amplifier module 32 detects andamplifies data read from memory cell transistors to a corresponding bitline. The sense amplifier module 32 temporarily stores read data DATread from the memory cell transistors, and transfers the read data DATstored therein to the input/output circuit 12. During a write operationof data, the sense amplifier module 32 temporarily stores write data DATtransferred from the input/output circuit 12. The sense amplifier module32 transfers the write data DAT to a corresponding bit line.

1.1.3.2 Configuration of Block

Next, a circuit configuration of the memory cell array 11 in the memorychip 10_n will be described with reference to FIG. 3. The memory cellarray 11 includes a plurality of blocks BLK0 to BLKm, as describedabove. A description will be given of a circuit configuration of only asingle block BLKr; however, the configurations of the other blocks aresimilar thereto.

FIG. 3 is a circuit diagram of a block BLKr in the memory cell array 11.The block BLKr includes, for example, a plurality of string units SU0,SU1, SU2, and SU3. The following description assumes that a “string unitSU” refers to each of the string units SU0 to SU3. Each of the stringunits SU0 to SU3 includes a plurality of NAND strings (or memorystrings) NS.

The NAND strings NS each include a plurality of memory cell transistorsMT0, MT1, MT2, . . . , and MT7, and select transistors ST1 and ST2. Forsimplicity of description, a case in which each NAND string NS includeseight memory cell transistors MT0 to MT7 and two select transistors ST1and ST2 will be described herein as an example. The followingdescription assumes that a “memory cell transistor MT” refers to each ofthe memory cell transistors MT0 to MT7.

The memory cell transistors MT0 to MT7 each include a control gate and acharge storage layer, and store data in a non-volatile manner. Thememory cell transistors MT0 to MT7 are coupled in series between asource of the select transistor ST1 and a drain of the select transistorST2.

The memory cell transistor MT is capable of storing one-bit data, ordata of 2 or more bits.

Gates of select transistors ST1 included in a string unit SU0 arecoupled to a select gate line SGD0. Similarly, gates of selecttransistors ST1 in the string units SU1 to SU3 are respectively coupledto select gate lines SGD1 to SGD3. The select gate lines SGD0 to SGD3are each independently controlled by the row decoder module 19.

Gates of select transistors ST2 included in the string unit SU0 arecoupled to a select gate line SGS. Similarly, gates of selecttransistors ST2 in the string units SU1 to SU3 are coupled to the selectgate line SGS. The gates of the select transistors ST2 in the stringunits SU0 to SU3 may be coupled to respective select gate lines SGS. Theselect transistors ST1 and ST2 are used to select a string unit SU invarious operations.

Control gates of memory cell transistors MT0 to MT7 included in theblock BLKr are respectively coupled to word lines WL0 to WL7. The wordlines WL0 to WL7 are each independently controlled by the row decodermodule 19.

Bit lines BL0 to BLi (where i is an integer equal to or greater than 0)are each coupled to a plurality of blocks BLK0 to BLKm, and arerespectively coupled to corresponding NAND strings NS in string units SUincluded in the block BLKr. That is, of a plurality of NAND strings NSarranged in a matrix pattern in the block BLKr, each of the bit linesBL0 to BLi is coupled to drains of select transistors ST1 of acorresponding set of NAND strings NS in the same column. A source lineSL is coupled to the blocks BLK0 to BLKm. That is, the source line SL iscoupled to sources of the plurality of select transistors ST2 includedin the block BLKr.

In short, each string unit SU includes a plurality of NAND strings NSthat are coupled to different bit lines BL and coupled to the sameselect gate line SGD. Moreover, the block BLKr includes a plurality ofstring units SU that share the same word lines WL. Furthermore, thememory cell array 11 includes a plurality of blocks BLK0 to BLKm thatshare the same bit lines BL.

The block BLKr is, for example, a unit of data erasure. That is, datastored in memory cell transistors MT included in the block BLKr iserased in a batch. Data may be erased in units of string units SU or insmaller units.

A plurality of memory cell transistors MT that share a word line WL inone string unit SU will be referred to as a “cell unit CU”. A set ofone-bit data items respectively stored in the memory cell transistors MTincluded in the cell unit CU will be referred to as a “page”. A cellunit CU varies in storage capacity according to the number of bits ofdata stored in the respective memory cell transistors MT. For example,the cell unit CU stores 1-page data when each memory cell transistor MTstores one-bit data, stores 2-page data when each memory cell transistorMT stores two-bit data, and stores 3-page data when each memory celltransistor MT stores three-bit data.

A write operation and a read operation are performed on the cell unit CUon a page-by-page basis. In other words, a read operation and a writeoperation are collectively performed on a plurality of memory celltransistors MT coupled to a single word line WL provided in a singlestring unit SU.

The string units provided in the block BLKr are not limited to SU0 toSU3, and the number of the string units may be designated as any number.The number of NAND strings NS included in each string unit SU, and thenumber of memory cell transistors and select transistors included ineach NAND string NS may also be designated as any number. Furthermore,each memory cell transistor MT may be ametal-oxide-nitride-oxide-silicon (MONOS) type, which uses an insulatingfilm as a charge storage layer, or a floating-gate (FG) type, which usesa conductive layer as a charge storage layer.

Next, a cross-sectional structure of the block BLKr will be describedwith reference to FIG. 4. FIG. 4 is a cross-sectional view of a partialregion of the block BLKr. As shown in FIG. 4, a p-well region 40P isprovided above a semiconductor substrate 40. A plurality of NAND stringsNS are provided above the p-well region 40P. Specifically, aninterconnect layer 41, eight interconnect layers 42, and an interconnectlayer 43 are sequentially stacked above the p-well region 40P. Theinterconnect layer 41 functions as a select gate line SGS. Theinterconnect layers 42 respectively function as word lines WL0 to WL7.The interconnect layer 43 functions as a select gate line SGD.Insulating layers (not shown) are provided between these stackedinterconnect layers.

A pillar-shaped conductor 44, which extends through the interconnectlayers 41, 42, and 43 to reach the p-well region 40P, is provided. Onthe side surface of the conductor 44, a gate insulating layer 45, acharge storage layer (insulating layer) 46, and a block insulating layer47 are sequentially provided. With these, the memory cell transistors MTand select transistors ST1 and ST2 are provided. The conductor 44functions as a current path of each NAND string NS, and is used as aregion in which a channel of each transistor is formed. The upper end ofthe conductor 44 is coupled to a metal interconnect layer 49 with a via48 interposed therebetween. The metal interconnect layer 49 functions asa bit line BL.

In a surface region of the p-well region 40P, an n⁺-type impuritydiffusion layer 40S is provided. A contact plug 50 is provided on thediffusion layer 40S. The contact plug 50 is coupled to a metalinterconnect layer 51. The metal interconnect layer 51 functions as asource line SL.

In the surface region of the p-well region 40P, a p⁺-type impuritydiffusion layer 40C is also provided. A contact plug 52 is provided onthe diffusion layer 40C. The contact plug 52 is coupled to a metalinterconnect layer 53. The metal interconnect layer 53 functions as awell interconnect CPWELL. The well interconnect CPWELL is used to applya potential to the conductor 44 via the p-well region 40P.

A plurality of configurations described above are arranged in thedirection perpendicular to the sheet of FIG. 4 (the depth direction),and a set of a plurality of NAND strings NS aligned in the depthdirection form a string unit SU.

The memory cell array 11 may have other configurations. For example, thememory cell array 11 may have the configuration described in U.S. patentapplication Ser. No. 12/407,403, entitled “THREE DIMENSIONAL STACKEDNONVOLATILE SEMICONDUCTOR MEMORY”, filed on Mar. 19, 2009. The memorycell array 11 may also have the configuration described in each of U.S.patent application Ser. No. 12/406,524, entitled “THREE DIMENSIONALSTACKED NONVOLATILE SEMICONDUCTOR MEMORY”, filed on Mar. 18, 2009; U.S.patent application Ser. No. 12/679,991, entitled “NON-VOLATILESEMICONDUCTOR STORAGE DEVICE AND METHOD OF MANUFACTURING THE SAME”,filed on Mar. 25, 2010; and U.S. patent application Ser. No. 12/532,030,entitled “SEMICONDUCTOR MEMORY AND METHOD FOR MANUFACTURING SAME”, filedon Mar. 23, 2009. The entire contents of these patent applications areincorporated herein by reference.

Data erasing can be performed in units of blocks BLK, or smaller units.The erasing method is described in, for example, U.S. patent applicationSer. No. 13/235,389, entitled “NONVOLATILE SEMICONDUCTOR MEMORY DEVICE”,filed on Sep. 18, 2011. The erasing method is also described in U.S.patent application Ser. No. 12/694,690, entitled “NON-VOLATILESEMICONDUCTOR STORAGE DEVICE”, filed on Jan. 27, 2010. Furthermore, themethod is described in U.S. patent application Ser. No. 13/483,610,entitled “NONVOLATILE SEMICONDUCTOR MEMORY DEVICE AND DATA ERASE METHODTHEREOF”, filed on May 30, 2012. The entire contents of these patentapplications are incorporated herein by reference.

1.1.3.3 Threshold Voltage Distributions of Memory Cell Transistors

Next, a relationship between data and threshold voltage distributionsthat may be observed in memory cell transistors MT according to thepresent embodiment will be described. FIG. 5A is a diagram showing arelationship between data and threshold voltage distributions that maybe observed in memory cell transistors MT. An example described hereinis a case in which a triple-level cell (TLC) method by which three-bitdata can be stored in a single memory cell transistor MT is adopted as astorage method of the memory cell transistors MT. The present embodimentis applicable to cases using other storage methods such as asingle-level cell (SLC) method by which one-bit data can be stored in asingle memory cell transistor MT, a multi-level cell (MLC) method bywhich two-bit data can be stored in a single memory cell transistor MT,a quad-level cell (QLC) method by which four-bit data can be stored in asingle memory cell transistor MT, etc.

3-bit data that a memory cell transistor MT can store is constituted bya lower bit, a middle bit, and an upper bit. In the case of a memorycell transistor MT storing three bits, the memory cell transistor MT maytake any of the eight states corresponding to a plurality of thresholdvoltages. The eight states are referred to as a state “Er”, a state “A”,a state “B” a state “C” a state “D”, a state “E”, a state “F”, and astate “G” in ascending order. The plurality of memory cell transistorsMT each belonging to any of the states “Er”, “A”, “B”, “C”, “D”, “E”,“F”, and “G” form threshold voltage distributions such as shown in FIG.5A.

For example, data “111”, “110”, “100”, “000”, “010”, “011”, “001”, and“101” are respectively allocated to the states “Er”, “A”, “B”, “C”, “D”,“E”, “F”, and “G”. Data alignment is expressed as “Z, Y, X” where “X”represents a lower bit, “Y” represents a middle bit, and “Z” representsan upper bit. Threshold voltage distributions and data allocation may beset freely.

In order to read data stored in a memory cell transistor MT which is atarget of a read, a state to which a threshold voltage of this memorycell transistor MT belongs is judged. To judge a state, read voltagesAR, BR, CR, DR, ER, FR, and GR are used.

The state “Er” corresponds to, for example, a state in which data hasbeen erased (an erasure state). The threshold voltages of memory celltransistors MT, which belong to the state “Er”, are lower than thevoltage AR and take on a negative value, for example.

The states “A” to “G” correspond to states in which a charge is injectedinto the charge storage layer and data is written into the memory celltransistor MT. The threshold voltages of memory cell transistors MT,which belong to the states “A” to “G”, take on a positive value, forexample. The threshold voltages of memory cell transistors MT, whichbelong to the state “A”, are higher than the read voltage AR and areequal to lower than the read voltage BR. The threshold voltages ofmemory cell transistors MT, which belong to the state “B”, are higherthan the read voltage BR and are equal to lower than the read voltageCR. The threshold voltages of memory cell transistors MT, which belongto the state “C”, are higher than the read voltage CR and are equal tolower than the read voltage DR. The threshold voltages of memory celltransistors MT, which belong to the state “D”, are higher than the readvoltage DR and are equal to lower than the read voltage ER. Thethreshold voltages of memory cell transistors MT, which belong to thestate “E”, are higher than the read voltage ER and are equal to lowerthan the read voltage FR. The threshold voltages of memory celltransistors MT, which belong to the state “F”, are higher than the readvoltage FR and are equal to lower than the read voltage GR. Thethreshold voltages of memory cell transistors MT, which belong to thestate “G”, are higher than the read voltage GR and are equal to lowerthan a read voltage VREAD.

The voltage VREAD is a voltage which is applied to the word lines WLconnected to the memory cell transistors MT in the cell unit CU, whichis not a target of a read, and is higher than the threshold voltages ofthe memory cell transistors MT in any state. Accordingly, when thevoltage VREAD is applied to a control gate of a memory cell transistorMT, this memory cell transistor MT is turned on regardless of datastored therein.

A verify voltage used in a write operation is set between neighboringthreshold voltage distributions. Specifically, verify voltages AV, BV,CV, DV, EV, FV, and GV are respectively set in correspondence with thestates “A”, “B”, “C”, “D”, “E”, “F”, and “G”. For example, the verifyvoltages AV, BV, CV, DV, EV, FV, and GV are set to voltages slightlyhigher than the read voltages AR, BR, CR, DR, ER, FR, and GR,respectively.

As described above, each memory cell transistor MT can be set to any ofthe eight states, and is capable of storing three-bit data. A write anda read are performed on a page-by-page basis in a single cell unit CU.In the case where three-bit data is stored in each memory celltransistor MT, each lower bit, each middle bit, and each upper bit arerespectively allocated to three pages in a single cell unit CU. Withrespect to a lower bit, a middle bit, and an upper bit, a page that iswritten by one write operation or is read by one read operation, thatis, a set of lower bits, a set of middle bits, and a set of upper bitsheld in a cell unit CU are respectively referred to as a lower page, amiddle page, and an upper page.

In the case where the data allocation described above is applied, thelower page is confirmed by a read operation using the read voltages ARand ER. The middle page is confirmed by a read operation using the readvoltages BR, DR, and FR. The upper page is confirmed by a read operationusing the read voltages CR and GR.

1.2 Operation

An erase operation in the memory system 1 according to the firstembodiment will be described below. The erase operation is an operationto set memory cells to an erasure state. In other words, the eraseoperation is an operation in which electrons stored in a charge storagelayer of each memory cell transistor MT is drawn out, thereby causingthe threshold voltage of the memory cell transistor MT to transition toa threshold voltage distribution corresponding to the state “Er”.

FIG. 5B is a diagram showing the threshold voltage distributions ofmemory cell transistors MT after the erase operation. When the eraseoperation is performed on the memory cell transistors MT, the erasurestate of memory cell transistors MT which belong to the state “Er”transitions to any of an insufficient erasure state, an appropriateerasure state, and an excessive erasure state.

The insufficient erasure state is a state in which, as shown by (a) inFIG. 5B, the state “Er” enters into the tail of the state “A” in thethreshold voltage distributions, and in which drawing out of electronsstored in a charge storage layer of each memory cell transistor MT isinsufficient. The appropriate erasure state is a state in which, asshown by (b) in FIG. 5B, the state “Er” and the state “A” are separatedfrom each other by an appropriate interval in the threshold voltagedistributions, and in which the amount of electrons stored in a chargestorage layer of each memory cell transistor MT is appropriate. Theexcessive erasure state is a state in which, as shown by (c) in FIG. 5B,the state “Er” and the state “A” are separated from each other by aninterval greater than an appropriate interval, and in which drawing outof electrons stored in a charge storage layer of each memory celltransistor MT is excessive.

After the erase operation, the erasure state of memory cell transistorsMT is any of the insufficient erasure state, the appropriate erasurestate, and the excessive erasure state. The erasure state of memorytransistors MT after the erase operation is defined as the degree oferasure.

Damage caused to a memory cell can be reduced by preventing thetransition of memory cell transistors MT into the excessive erasurestate, in other words, preventing the memory cell transistors MT fromincreasing in degree of erasure. Furthermore, read errors can be reducedin page reads by preventing the transition of memory cell transistors MTinto the insufficient erasure state, in other words, preventing thememory cell transistors MT from decreasing in degree of erasure.

1.2.1 Erase Operation of Memory System

The erase operation of data stored in the semiconductor memory device 10is executed under an erase instruction output from the memory controller20 to the semiconductor memory device 10. The erase operation in thesemiconductor memory device 10 can be performed, for example, in unitsof blocks or smaller units, as described above. Herein, a case in whichthe erase operation is performed in units of blocks is described as anexample.

Hereinafter, a basic data erase operation in the memory system 1 will bedescribed. FIG. 6 is a diagram showing the basic erase operation in thememory system 1. In FIG. 6, a command output in the form of I/O signalsDQ0 to DQ7 from the memory controller 20 is expressed by a hexagon,whereas an address is expressed by a rounded corner square (or anellipse).

As shown in FIG. 6, the memory controller 20 outputs, to thesemiconductor memory device 10, an erase setup command “60h”, an address“ADD” of an erasure target block, and thereafter an erase executioncommand “D0h”. The sequencer 16 acknowledges receipt of an eraseinstruction by the command register 15C storing the erase setup command“60h”. Furthermore, in response to receipt of the erase executioncommand “D0h”, the sequencer 16 starts the erase operation. As shown inFIG. 6, the erase operation includes data erase processing and eraseverify processing. In tandem with the start of the erase operation, thesequencer 16 causes the ready/busy signal R/Bn to transition from theready state to the busy state (R/Bn=“L”). In a command (or an address),the letter “h” assigned thereto indicates that a corresponding value ishexadecimal.

The erase operation is an operation to erase data stored in memory celltransistors MT in an erasure target block. More specifically, the eraseoperation is an operation in which electrons are drawn out from a chargestorage layer of each memory cell transistor MT in an erasure targetblock by applying the erase voltage VERA to a well interconnect CPWELL.A pulse time of the erase voltage VERA to be applied may be set to apredetermined value. The pulse time is a time during which a voltagelevel of the erase voltage VERA is maintained, and is also referred toas a pulse width or a pulse length. The erase verify processing is anoperation to verify erasure of data in memory cell transistors MT by theerase operation. In other words, the erase verify processing is anoperation to confirm whether or not a threshold voltage of each memorycell transistor MT has transitioned to a threshold voltage in theerasure state.

In the erase operation, a single erase loop consists of erase processingand erase verify processing performed after the erase processing. Theexample in FIG. 6 shows a first verify loop and a second verify loop. Inverification by the erase verify processing, if the number of memorycell transistors MT, which have a threshold voltage higher than acertain value, is less than a predetermined number, it is judged that anerasure target block has passed the erase verify. On the other hand, ifthe number of memory cell transistors MT, which have a threshold voltagehigher than a certain value, is more than a predetermined number, it isjudged that an erasure target block has failed the erase verify. Uponcompletion of these judgments, the sequencer 16 terminates the eraseverify processing. A pair of the erase processing and the erase verifyprocessing described above corresponds to a single erase loop. If anerasure target block fails the erase verify, the sequencer 16 repeatsthe erase loop. If the block passes the erase verify, the sequencer 16terminates the erase operation.

Electrons injected into a charge storage layer of each memory celltransistor MT drop below a predetermined number through a plurality oferase loops, not in one stroke by the first erase loop. For example, asshown in FIG. 6, if an erase verify in the first erase loop fails, thesecond erase loop is carried out. The erase voltage VERA increased byAVERA is set every time erase processing is performed in the repeatederase loop. When an erasure target block passes the erase verify and theerase operation is terminated, the sequencer 16 causes the semiconductormemory device 10 to transition from the busy state to the ready state.After this transition to the ready state, the erase processing isterminated.

FIG. 7 is a diagram showing voltage waveforms of respective signals inthe erase operation.

First, the erase processing is executed from time t0 to time t5, andthereafter, the erase verify processing is executed from time t5 to timet10. This processing sequence from t0 to t10 corresponds to a singleerase loop.

Hereinafter, the erase processing will be described. At time to, a bitline BL, select gate lines SGD and SGS, word lines WL, a source lineSELSRC, and a well interconnect CPWELL are set to a voltage VSS (forexample, 0 V).

Next, from time t1 to time t3, for example, the driver 18 applies theerase voltage VERA to the well interconnect CPWELL. Accordingly, fromtime t1 to time t3, a channel region of a memory transistor MT isincreased to the erase voltage VERA. Furthermore, because of capacitancecoupling caused by the voltage VERA applied to the well interconnectCPWELL, the bit line BL, the select gate lines SGD and SGS, word linesWL of blocks which are not an erasure target (or non-selected blocks),and the source line SELSRC are increased to the voltage VERA. The selectgate line SGS is increased to a voltage lower by a voltage Δ than thevoltage VERA.

From time t1 to time t3, furthermore, the row decoder module 19 appliesa voltage Vwl lower than the erase voltage VERA to word lines WL of anerasure target block (or a selected block). This causes a potentialdifference between the erase voltage VERA in the channel region of eachmemory cell transistor MT and the voltage Vwl of word lines WL of anerasure target block, so that electrons are drawn out from a chargestorage layer to a channel layer in each memory cell transistor MT inthe erasure target block. That is, data in the memory cell transistorsMT in the erasure target block is erased.

Thereafter, from time t3 to time t5, the bit line BL, the select gatelines SGD and SGS, the word lines WL, the source line SELSRC, and thewell interconnect CPWELL are set to the voltage VSS. The erase operationis thus completed.

Next, the erase verify processing from time t5 to time t10 will bedescribed.

At time t6, the row decoder module 19 applies a voltage VSG to theselect gate lines SGD and SGS of the selected block. The voltage VSG isa voltage that sets the select transistors ST1 and ST2 to the ON state.

Next, from time t7 to time t9, the row decoder module 19 applies anerase verify voltage Vev to the word lines WL of the erasure targetblock. The row decoder module 19 further applies the voltage VREAD toword lines WL of blocks which are not an erasure target. The eraseverify voltage Vev is a read voltage for judging the erasure state ofeach memory cell transistor MT in an erasure target block.

With this, the sense amplifier module 32 senses and amplifies data readout to the bit line BL. In accordance with a result of this read, thesequencer 16 judges whether or not the erase operation on an erasuretarget block has been completed, that is, whether the block has passedor failed the erase verify. If the erase operation is not completed, theerase operation including the erase processing and the erase verifyprocessing is repeatedly executed on the erasure target block.

1.2.2 Erase Operation in First Embodiment

In the erase operation according to the first embodiment, the pulse timeof the erase voltage VERA is adjusted (or changed) based on an eraseresult of memory cells after an erase operation or based on an eraseresult of memory cells after a write operation subsequent to the eraseoperation. For example, the pulse time of the erase voltage VERA isextended or shortened. The erase result of memory cells indicates adetermination result about the degree of erasure in memory cells afterthe erase operation. In other words, the erase result of memory cellsindicates any of the insufficient erasure state, the appropriate erasurestate, and the excessive erasure state that memory cells have after theerase operation.

1.2.2.1 First Example of Erase Operation

In the first example, after the erase operation, the pulse time of theerase voltage VERA is updated based on the erase result of memory cellsbelonging to a word line WL and a string unit SU which are a measurementtarget in an erasure target block. An example described herein is a casein which the pulse time of the erase voltage VERA is extended based onan erase result of memory cells.

FIG. 8 is a flowchart showing a first example of the erase operation inthe memory system 1 according to the first embodiment. FIG. 9 is adiagram showing operations performed between the memory controller 20and the semiconductor memory device 10. FIG. 10 is a diagram showing anexample of a pulse time management table 22B_1 provided in the memory 22of the memory controller 20. The pulse time management table 22B_1manages the pulse time of the erase voltage VERA. In the pulse timemanagement table 22B_1, a block BLKr (where r is an integer equal to orgreater than 0 and equal to or less than m) is associated with a pulsetime PDr used for the erase operation on this block BLKr. The processingshown in FIG. 8 is instructed and controlled by the memory controller 20(or the CPU 21).

As shown in FIGS. 8 and 9, when the erase operation is started, first,the memory controller 20 transmits to the semiconductor memory device 10a pulse time PDr of the erase voltage VERA corresponding to the erasuretarget block BLKr in the pulse time management table 22B_1, therebysetting the pulse time PDr to the register 15D_1 of the semiconductormemory device 10. More specifically, the memory controller 20 obtainsfrom the pulse time management table 22B_1 the pulse time PDr of theerase voltage VERA corresponding to the erasure target block BLKr,transmits the obtained pulse time PDr to the semiconductor memory device10, and causes the register 15D_1 to store the pulse time PDr (step S1).For example, when an erasure target block is the block BLK0, the memorycontroller 20 transmits a pulse time PD0 associated with the block BLK0to the semiconductor memory device 10, thereby causing the register15D_1 to store the pulse time PD0.

Next, the memory controller 20 instructs the semiconductor memory device10 to perform the erase operation (step S2). The sequencer 16 of thesemiconductor memory device 10 executes the erase operation on theerasure target block BLKr using the pulse time PDr of the erase voltageVERA stored in the register 15D_1.

Next, after the semiconductor memory device 10 transitions to the readystate, the memory controller 20 obtains from the semiconductor memorydevice 10 an erase result of the erase operation on memory cells. Basedon the erase result of the memory cells obtained from the semiconductormemory device 10, the memory controller 20 determines whether to updatethe pulse time PDr of the erase voltage VERA (hereinafter, thisdetermination is also described as “determining the pulse time of theerase voltage VERA”) (step S3). For example, the processing of“determining the pulse time of the erase voltage VERA” in step S3 isperformed every time the erase operation is executed, every time thewrite operation/erase operation is executed a predetermined number oftimes, or when the number of times the write operation/erase operationis executed reaches a predetermined number. The processing of“determining the pulse time of the erase voltage VERA” in step S3 willbe described later in detail.

Next, based on a determination result of “determining the pulse time ofthe erase voltage VERA” in step S3, the memory controller 20 updates ormaintains without updating the pulse time PDr associated with theerasure target block BLKr in the pulse time management table 22B_1 ofthe memory 22 (step S4). The erase operation is thus completed.

Next, the processing of “determining the pulse time of the erase voltageVERA” in step S3 in the flowchart shown in FIG. 8 will be described indetail. FIG. 11A is a flowchart showing the processing of “determiningthe pulse time of the erase voltage VERA” in step S3. FIG. 12 is adiagram showing a judgment level and a threshold voltage distributionthat shows a method of determining the degree of erasure in the firstexample of the erase operation. The processing shown in FIG. 11A isinstructed and controlled by the memory controller 20 (or the CPU 21).

First, the read operation for determining the degree of erasure inmemory cells after the erase operation is performed. As shown in stepS11 in FIG. 11A, the memory controller 20 sets the read voltage AR ofthe state “A” as a read voltage, and further sets a shift value (voltagevalue) used for shifting the read voltage AR toward a low voltage side.As shown in FIG. 12, a read voltage shifted from the read voltage AR bythe shift value is defined as a judgment level AR2. The judgment levelAR2 is a voltage level used for determining the degree of erasure inmemory cells after the erase operation. The memory controller 20 sets tothe semiconductor memory device 10 the judgment level AR2 shifted fromthe read voltage AR by the shift value. Depending on the degree oferasure in memory cells, the read voltage AR may be used as the judgmentlevel AR2 by making no change or by making a shift toward a high voltageside. Furthermore, the memory controller 20 designates a word line WLand a string unit SU both serving as a measurement target in an erasuretarget block (step S11). The number of measurement target word lines WLmay be one or more, or all of the word lines WL may be a measurementtarget.

The memory controller 20 instructs the semiconductor memory device 10 toperform a “one-level read” of the state “A” (step S12). The “one-levelread” is processing for acquiring read data indicative of the magnitudeof threshold voltage with respect to the read voltage corresponding toone state. For example, in the case of TLC, the read voltagecorresponding to any of the states “A” to “G” is designated. Herein, theread operation is executed using the judgment level AR2 obtained byshifting the read voltage AR by the shift value. Upon receipt of theinstruction for the “one-level read” of the state “A”, the sequencer 16of the semiconductor memory device 10 executes the read operation usingthe set judgment level AR2 on measurement target memory cells. In thisread operation, as shown in FIG. 12, a memory cell having a thresholdvoltage higher than the judgment level AR2 transitions to the OFF state,not into the ON state. The sequencer 16 outputs a read result RA2R ofthe read operation using the judgment level AR2, from the semiconductormemory device 10 to the memory controller 20. As an erase result afterthe erase operation, the memory controller 20 counts the number “DO1” ofmemory cells in the OFF state (hereinafter, referred to as the number ofOFF bits). The number of OFF bits is stored in the buffer 22A in thememory 22.

Next, based on the read result of the read operation using the judgmentlevel AR2, the memory controller 20 determines the degree of erasure inmemory cells after the erase operation (step S13). That is, the memorycontroller 20 determines the degree of erasure in memory cells after theerase operation, from the number of OFF bits obtained through the readoperation using the judgment level AR2. More specifically, the memorycontroller 20 executes the read operation using the judgment level AR2either every time the erase operation is executed on an erasure targetblock BLKr or at the ratio of one read operation to several eraseoperations. In the erase operation, when the number of times the readoperation is performed at the judgment level AR2 has reached the numberX (for example, 4), the memory controller 20 determines whether or notthe average number of OFF bits in the read operation performed thenumber X times exceeds a reference value Y1 (step S14). In the casewhere the average number of OFF bits exceeds the reference value Y1(Yes), the memory controller 20 extends the pulse time PDr of the erasevoltage VERA by a predetermined time length (step S15). On the otherhand, if the average number of OFF bits does not exceed the referencevalue Y1 (No), the memory controller 20 terminates the processing of“determining the pulse time of the erase voltage VERA”.

In step S14, the average number of OFF bits in the read operationperformed the number X times is compared with the reference value Y1.However, the maximum number of OFF bits on one or more word lines WLwhen the read operation is performed the number X times may be comparedwith the reference value Y1. Alternatively, the maximum number of OFFbits on all word lines WL when the read operation is performed thenumber X times may be compared with the reference value Y1.

The reference value Y1 may be changed depending on the number of timesthe write operation/erase operation is performed on memory cells. FIG.11B shows an example of a relationship between the number of times thewrite operation/erase operation is performed and the reference value Y1.As shown in FIG. 11B, the reference value Y1 may be decreased as thenumber of times the write operation/erase operation is performed onmemory cells increases. More specifically, the reference value Y1 may bedecreased in stages every time the number of times the writeoperation/erase operation is performed increases by a predeterminednumber. The increased number of times the write operation/eraseoperation is performed encourages the wearing out of memory cells. Bysetting the reference value Y1 in accordance with an increase in thenumber of times the write operation/erase operation is performed, thedegree of erasure in memory cells can be stabilized in smaller units.

Next, the determination as to whether the number of OFF bits in step S14exceeds the reference value Y1 will be described with reference to FIGS.13 and 14. That is, a specific example of the determination of whetherto update the pulse time PDr is described. FIGS. 13 and 14 are diagramsshowing the number of OFF bits stored in the buffer 22A at the time whenmeasurement target memory cells are read using the judgment level AR2.Herein, t-3 represents the number of OFF bits obtained through the readoperation performed three times before, t-2 represents the number of OFFbits obtained through the read operation performed two times before, andt-1 represents the number of OFF bits obtained through the readoperation performed one time before (that is, the last time).Furthermore, t-0 represents the number of OFF bits obtained through thelatest read operation (the read operation this time). Herein, thereference value Y1 is set to, for example, 30.

FIG. 13 shows an example in which t-3, t-2, and t-1 of the buffer 22Arespectively store 29, 35, and 29 as the number of bits, and t-0 stores23 as the latest number of OFF bits. In this example case, the averagenumber of OFF bits stored in t-3, t-2, t-1, and t-0 is equal to 29.Since the average number of OFF bits is 29, which is below the referencevalue of 30, the memory controller 20 terminates the processing withoutchanging the pulse time PDr of the erase voltage VERA. For example, theCPU 21 of the memory controller 20 is equipped with a circuit configuredto calculate the pulse time PDr using the average number of OFF bits andthe reference value. This allows the memory controller 20 to calculatethe pulse time PDr from the average number of OFF bits and to reflectthe calculated pulse time PDr in the pulse time management table 22B_1of the memory 22.

Furthermore, at the time of termination, the memory controller 20deletes the earliest number of OFF bits stored in t-3 of the buffer 22A,and sequentially shifts the values in t-2, t-1, and t-0. After shifting,t-3, t-2, and t-1 of the buffer 22A respectively store 35, 29, and 23.

FIG. 14 shows an example in which t-3, t-2, and t-1 of the buffer 22Arespectively store 29, 35, and 29 as the number of bits, and t-0 stores32 as the latest number of OFF bits. In this example case, the averagenumber of OFF bits stored in t-3, t-2, t-1, and t-0 is equal to 31.Since the average number of OFF bits is 31, which exceeds the referencevalue of 30, the memory controller 20 extends the pulse time PDr of theerase voltage VERA by a predetermined time length, and then terminatesthe processing.

At the time of termination, the memory controller 20 clears all of theset3, t2, and t1 of the buffer 22A.

Described next is input/output of commands, addresses, and data betweenthe memory controller 20 and the semiconductor memory device 10 in thefirst example of the erase operation described above.

FIG. 15 is a diagram showing a command sequence in the first example ofthe erase operation according to the first embodiment. The commandsequence shown in FIG. 15 includes an input/output cycle of commands,addresses, and data. A command is expressed by a hexagon, an address isexpressed by a rounded corner square (or an ellipse), and a datainput/output cycle is expressed by a square. Output of commands andaddresses from the memory controller 20 to the semiconductor memorydevice 10 and input/output of data between the memory controller 20 andthe semiconductor memory device 10, which will be described below, areperformed using the I/O signals DQ0 to DQ7.

As shown in FIG. 15, the command sequence includes: a phase P1corresponding to “setting the pulse time of the erase voltage VERA (stepST)”; a phase P2 corresponding to “instructing the erase operation (stepS2)”; a phase P3 corresponding to “setting the judgment level AR2 (stepS11)” for a shift read in the one-level read; and phases P3 and P4corresponding to “instructing the one-level read (step S12)” in theshift read.

First, in the phase P1 for setting the pulse time of the erase voltageVERA, the memory controller 20 sequentially outputs a command “0Xh”, anaddress “00h”, and data “PDr” to the semiconductor memory device 10. Thecommand “0Xh” is a command for designating an erase mode. The address“00h” is an address for setting the pulse time of the erase voltageVERA. The data “PDr” is data indicative of the pulse time of the erasevoltage VERA corresponding to an erasure target block BLKr and is storedin the register 15D_1. In this manner, the memory controller 20designates an erase mode for the semiconductor memory device 10, andsets to the register 15D_1 of the semiconductor memory device 10 thepulse time PDr of the erase voltage VERA used in the erase operation onthe erasure target block BLKr.

Next, in the phase P2 for instructing the erase operation, the memorycontroller 20 sequentially outputs an erase setup command “60h”, anaddress “ADD” of the erasure target block BLKr, and the erase executioncommand “D0h” to the semiconductor memory device 10. In this manner, thesequencer 16 executes the erase operation on the erasure target blockBLKr by applying the erase voltage VERA having the pulse time PDr to thewell interconnect CPWELL. During the execution of this erase operation,the sequencer 16 causes the ready/busy signal R/Bn to transition fromthe ready state to the busy state (R/Bn=“L”).

Thereafter, the memory controller 20 outputs a status read command “70h”to the semiconductor memory device 10. Upon receipt of the status readcommand “70h”, the semiconductor memory device 10 outputs to the memorycontroller 20 data indicative of whether an erasure target block haspassed or failed the erase operation. Herein, for example, thesemiconductor memory device 10 outputs to the memory controller 20 data“PASS” indicating that an erasure target block has passed the eraseoperation.

Next, in the phase P3 for setting the judgment level AR2, the memorycontroller 20 sequentially outputs to the semiconductor memory device 10a one-level read command “X1h” and an address “01h” indicative of theread voltage AR of the state “A”. Furthermore, the memory controller 20sequentially outputs, to the semiconductor memory device 10, a shiftread command “X2h”, an address “01h”, and data “SHIFT” indicative of ashift value from the read voltage AR. In this manner, the memorycontroller 20 sets to the semiconductor memory device 10 the judgmentlevel AR2 used for the shift read in the one-level read.

Next, in the phase P4 for instructing the read, the memory controller 20sequentially outputs to the semiconductor memory device 10 a read setupcommand “00h”, addresses “ADD1 to ADD5” of measurement target memorycells, and a read execution command “30h” for instructing the start ofthe read operation. Upon receipt of the read execution command “30h”,the sequencer 16 of the semiconductor memory device 10 executes the readoperation using the judgment level AR2 on measurement target memorycells, which are designated by the addresses “ADD1 to ADD5”. During theexecution of this read operation, the sequencer 16 causes the ready/busysignal R/Bn to transition from the ready state to the busy state(R/Bn=“L”). Thereafter, the sequencer 16 outputs a read result RA2R ofthe read operation using the judgment level AR2, from the semiconductormemory device 10 to the memory controller 20. The memory controller 20counts the number “DO1” (that is, the number of OFF bits) of memorycells which remain in the OFF state without transitioning to the ONstate.

As described above, in the first example, through the read operationafter the erase operation, the memory controller 20 counts the number ofOFF bits among measurement target memory cells in the erasure targetblock BLKr. The memory controller 20, based on the number of OFF bits,updates the pulse time PDr of the erase voltage VERA or maintainswithout updating the pulse time PDr of the erase voltage VERA.

<First Modification of First Example>

Next, a modification of the first example of the erase operationaccording to the first embodiment will be described. The presentmodification describes an example in which the pulse time of the erasevoltage VERA is shortened or extended based on the erase result ofmemory cells after an erase operation.

The erase operation in the modification is executed in accordance withthe flowchart of the erase operation shown in FIG. 8 just as with thefirst example described above. The processing in step S3 in theflowchart shown in FIG. 8 is replaced with the processing shown in FIG.16. FIG. 16 is a flowchart showing the processing of “determining thepulse time of the erase voltage VERA” (step S3) in the modification. Theprocessing shown in FIG. 16 is instructed and controlled by the memorycontroller 20 (or the CPU 21).

The processing from steps S11 to S15 shown in FIG. 16 is similar to theprocessing from steps S11 to S15 shown in FIG. 11A described above. Thatis, the memory controller 20 sets to the semiconductor memory device 10,the judgment level AR2 used to determine the degree of erasure in memorycells after the erase operation. Furthermore, the memory controller 20designates a word line WL and a string unit SU serving as a measurementtarget in an erasure target block BLKr (step S11).

Next, the memory controller 20 instructs the semiconductor memory device10 to perform the “one-level read” (step S12). Upon receipt of theinstruction for the “one-level read”, the sequencer 16 of thesemiconductor memory device 10 executes the read operation at the setjudgment level AR2 on, for example, measurement target memory cells.

Next, based on the number of OFF bits calculated from a read result RA2Rof the read operation using the judgment level AR2, the memorycontroller 20 determines the degree of erasure in memory cells after theerase operation (step S13). More specifically, the memory controller 20executes the read operation using the judgment level AR2 in the case ofdetermining the degree of erasure in memory cells after the eraseoperation. When the number of times the read operation is performed atthe judgment level AR2 has reached the number X, the memory controller20 determines whether or not the average number of OFF bits in the readoperation performed the number X times exceeds the reference value Y1(step S14). If it is determined in step S14 that the average number ofOFF bits in the read operation performed the number X times exceeds thereference value Y1 (Yes), the memory controller 20 extends the pulsetime PDr of the erase voltage VERA by a predetermined time length (stepS15), and terminates the processing. On the other hand, if the averagenumber of OFF bits in the read operation performed the number X timesdoes not exceed the reference value Y1 (No), the memory controller 20determines whether or not the average number of OFF bits in the readoperation performed the number X times has reached the reference valueY2 (step S16).

If it is determined in step S16 that the average number of OFF bits inthe read operation performed the number X times has not reached thereference value Y2 (No), the memory controller 20 shortens the pulsetime PDr of the erase voltage VERA by a predetermined time length (stepS17), and terminates the processing. On the other hand, if the averagenumber of OFF bits in the read operation performed the number X timeshas reached the reference value Y2 (Yes), the memory controller 20terminates the processing without updating the pulse time of the erasevoltage VERA.

As described above, in the modification of the first example, throughthe read operation after the erase operation, the number of OFF bitsamong measurement target memory cells in the erasure target block BLKris output from the semiconductor memory device 10 to the memorycontroller 20. Based on the number of OFF bits, the memory controller 20determines which one of the insufficient erasure state, the appropriateerasure state, and the excessive erasure state the memory cells afterundergoing the erase operation are in. In the case of memory cells beingin the excessive erasure state (No in step S16), the memory controller20 shortens the pulse time PDr of the erase voltage VERA (step S17). Inthe case of memory cells being in the appropriate erasure state (Yes instep S16), the memory controller 20 does not update the pulse time PDr.In the case of memory cells being in the insufficient erasure state (Yesin step S14), the memory controller 20 extends the pulse time PDr (stepS15).

1.2.2.2 Second Example of Erase Operation

In the second example, after the erase operation subsequent to the eraseoperation, the pulse time of the erase voltage VERA is updated based onthe erase result of memory cells belonging to a word line WL and astring unit SU serving as a measurement target in an erasure targetblock. An example described herein is a case in which the pulse time isextended. In the second example, the write operation is added betweenthe erase operation and the determination as to whether to update thepulse time.

FIG. 17 is a flowchart showing the second example of the erase operationin the memory system 1 according to the first embodiment. The processingshown in FIG. 17 is instructed and controlled by the memory controller20 (or the CPU 21).

As with the first example, the memory controller 20 transmits to thesemiconductor memory device 10 the pulse time PDr of the erase voltageVERA corresponding to the erasure target block BLKr in the pulse timemanagement table 22B_1, thereby setting the pulse time PDr to theregister 15D_1 of the semiconductor memory device 10 (step S1). Thememory controller 20 instructs the semiconductor memory device 10 toperform the erase operation (step S2). Upon receipt of the instructionfor the erase operation, the sequencer 16 of the semiconductor memorydevice 10 executes the erase operation on the erasure target block BLKr.

After execution of the erase operation on the erasure target block BLKr,the memory controller 20 instructs the semiconductor memory device 10 toexecute the write operation (step S5). Upon receipt of the writeoperation, the sequencer 16 of the semiconductor memory device 10executes the write operation on memory cells which are a target of awrite in the erasure target block BLKr. The instruction for the writeoperation is repeated until when a write in a part of the block iscompleted or a write in the entire block is completed (step S6).

Next, based on the erasure state of memory cells after the writeoperation, the memory controller 20 determines whether to update thepulse time PDr of the erase voltage VERA (step S3A). For example, aswith the first example, the processing of “determining the pulse time ofthe erase voltage VERA” in step S3A is performed every time the writeoperation is executed after the erase operation, or every time the writeoperation/erase operation is executed a predetermined number of times,or when the number of times the write operation/erase operation isexecuted reaches a predetermined number. The processing of “determiningthe pulse time of the erase voltage VERA” in step S3A will be describedlater in detail.

Next, based on a determination result of “determining the pulse time ofthe erase voltage VERA” in step S3A, the memory controller 20 updates ormaintains without updating the pulse time PDr associated with theerasure target block BLKr in the pulse time management table 22B_1 ofthe memory 22 (step S4). The erase operation is thus completed.

Next, the processing of “determining the pulse time of the erase voltageVERA” in step S3A in the flowchart shown in FIG. 17 will be described indetail. FIG. 18 is a flowchart showing the processing of “determiningthe pulse time of the erase voltage VERA” in step S3A. FIG. 19 is adiagram showing threshold voltage distributions and judgment levels, forexplaining a method of determining the degree of erasure in the secondexample of the erase operation, and data for calculating the number ofOFF bits. The processing shown in FIG. 18 is instructed and controlledby the memory controller 20 (or the CPU 21).

First, as shown in FIG. 18, the memory controller 20 sets a shift valuefor making a shift from the read voltage AR. Herein, as shown in FIG.19(A), a read voltage shifted from the read voltage AR by the shiftvalue is defined as a judgment level AR3. The judgment level AR3 is avoltage level used for determining the degree of erasure in memory cellsafter the write operation. The memory controller 20 sets to thesemiconductor memory device 10 the judgment level AR3 shifted from theread voltage AR by the shift value. Depending on the degree of erasurein memory cells, the read voltage AR may be used as the judgment levelAR3 by making no change or by making a shift toward a high voltage side.Furthermore, the memory controller 20 designates a word line WL and astring unit SU serving as a measurement target in an erasure targetblock BLKr (step S21). The number of measurement target word lines WLmay be one or more, or all of the word lines WL may be a measurementtarget. The following description assumes a read based on memory mappingshown in FIG. 5A. In FIG. 5A, the read voltage AR is mapped on a read onthe “lower page”.

Next, the memory controller 20 instructs the semiconductor memory device10 to execute a “lower page read” including the read voltage AR (stepS22). The “lower page read” is to read data from a lower page by theread operation using, for example, the read voltages AR and ER. Herein,the judgment level AR3 is used instead of the read voltage AR. Uponreceipt of the instruction for the “lower page read”, the sequencer 16of the semiconductor memory device 10 executes the read operation usingthe set judgment level AR3 and read voltage ER on, for example,measurement target memory cells, and obtains lower page data RLP beforebeing subjected to error correction shown in FIG. 19(B). The lower pagedata RLP is data immediately after a read using the judgment level AR3and the read voltage ER, and is not yet subjected to the errorcorrection.

Next, the memory controller 20 receives from the semiconductor memorydevice 10 the lower page data RLP before being subjected to the errorcorrection, obtained through the read operation using the judgment levelAR3 and the read voltage ER. The memory controller 20 causes the ECCcircuit 24 to correct an error in the lower page data RLP before beingsubjected to the error correction, and obtains the lower page data CLPafter being subjected to the error correction shown by (B) in FIG. 19(step S23).

Next, the memory controller 20 separates data read at only the judgmentlevel AR3 from the read lower page data. For example, the memorycontroller 20 sets the read voltage CR of the state “C” to thesemiconductor memory device 10. In this embodiment, a set state is notlimited to the state “C”, and may be any of the states B, C, and D whichenable the separation between the states “A” and “E”. The memorycontroller 20 further designates to the semiconductor memory device 10 aword line WL and a string unit SU serving as a measurement target in anerasure target block BLKr (step S24).

Next, the memory controller 20 instructs the semiconductor memory device10 to perform the “one-level read” of the state “C (step S25)”. Uponreceipt of the instruction for the “one-level read”, for example, thesequencer 16 of the semiconductor memory device 10 executes the readoperation at the read voltage CR on measurement target memory cells, andobtains a read result RCR of the state “C” shown by (B) in FIG. 19.

The memory controller 20 performs an AND operation on the data RLP2 andtwo data pieces CLP and RCR, thereby counting the number of OFF bits todetermine the degree of erasure. Herein, the data RLP2 is data obtainedby performing a NOT operation on the lower page data RLP before beingsubjected to the error correction, obtained in step S22, and the twodata pieces CLP and RCR are data obtained in steps S23 and S25. Thememory controller 20 stores the obtained number of OFF bits in thebuffer 22A within the memory 22.

Next, based on the erasure state of memory cells after the writeoperation, the memory controller 20 determines the degree of erasure inmemory cells which are a target of erasure after the write operation.That is, the memory controller 20 determines the degree of erasure inmemory cells which are a target of erasure after the write operation,from the number of OFF bits stored in the buffer 22A (step S26). Morespecifically, in the case of determining the degree of erasure after thewrite operation subsequent to the erase operation, the memory controller20 executes the processing in steps S22, S23, and S25 on measurementtarget memory cells. When the number of times the processing in stepsS22, S23, and S25 is performed has reached the number X (for example,4), the memory controller 20 determines whether or not the averagenumber of OFF bits in the aforementioned processing performed the numberX times exceeds a reference value Y1 (step S27). In the case where theaverage number of OFF bits in the processing performed the number Xtimes exceeds the reference value Y1 (Yes), the memory controller 20extends the pulse time PDr of the erase voltage VERA by a predeterminedtime length (step S28). On the other hand, if the average number of OFFbits in the processing performed the number X times does not exceed thereference value Y1 (No), the memory controller 20 terminates theprocessing of “determining the pulse time of the erase voltage VERA”.

In step S27, the average number of OFF bits in the aforementionedprocessing performed the number X times is compared with the referencevalue Y1. However, the maximum number of OFF bits on one or more wordlines WL when the aforementioned processing is performed the number Xtimes may be compared with the reference value Y1. Alternatively, themaximum number of OFF bits on all word lines WL when the aforementionedprocessing is performed the number X times may be compared with thereference value Y1.

The determination as to whether the number of OFF bits in step S27described above exceeds the reference value Y1, that is, a specificexample of the determination as to whether to update the pulse time, issimilar to that of the first example. In the case where the averagenumber of OFF bits stored in t-3, t-2, t-1, and t-0 of the buffer 22Adoes not exceed the reference value of 30, the memory controller 20terminates the processing without changing the pulse time of the erasevoltage VERA. On the other hand, if the average number of OFF bitsstored in t-3, t-2, t-1, and t-0 of the buffer 22A exceeds the referencevalue of 30, the memory controller 20 extends the pulse time of theerase voltage VERA by a predetermined time length, and then terminatesthe processing.

The following description is about input/output of commands, addresses,and data between the memory controller 20 and the semiconductor memorydevice 10 in the second example of the erase operation according to thefirst embodiment.

FIG. 20 is a diagram showing a command sequence in the second example ofthe erase operation according to the first embodiment. The I/O signalsDQ0 to DQ7 are used for output of commands and addresses from the memorycontroller 20 to the semiconductor memory device 10 and for input/outputof data between the memory controller 20 and the semiconductor memorydevice 10, which will be described below. The command sequence shown inFIG. 20 includes an input/output cycle of commands, addresses, and data.A command is expressed by a hexagon, an address is expressed by arounded corner square (or an ellipse), and a data input/output cycle isexpressed by a square.

As shown in FIG. 20, the command sequence includes: a phase P1corresponding to “setting the pulse time of the erase voltage VERA (stepS1)”; a phase P2 corresponding to “instructing the erase operation (stepS2)”; a phase P5 corresponding to “instructing the write operation (stepS5)”; a phase P3A corresponding to “setting the judgment level AR3 (stepS21)” for a shift read in the lower page read; a phase P4A correspondingto “instructing the lower page read (step S22)” in the shift read; and aphase P6 corresponding to “instructing the one-level read (step S25)”.The phases P1 and P2 are similar to the above-described phases P1 and P2shown in FIG. 15.

First, in the phase P1 for setting the pulse time of the erase voltageVERA, the memory controller 20 sequentially outputs a command “0Xh”, anaddress “00h”, and data “PDr” to the semiconductor memory device 10. Inthis manner, the memory controller 20 designates an erase mode for thesemiconductor memory device 10, and sets to the register 15D_1 of thesemiconductor memory device 10 the pulse time PDr of the erase voltageVERA used in the erase operation on the erasure target block BLKr.

Next, in the phase P2 for instructing the erase operation, the memorycontroller 20 sequentially outputs an erase setup command “60h”, anaddress “ADD” of the erasure target block BLKr, and the erase executioncommand “D0h” to the semiconductor memory device 10. Upon receipt of theerase execution command “D0h”, the sequencer 16 of the semiconductormemory device 10 executes the erase operation on the erasure targetblock BLKr by applying the erase voltage VERA of the pulse time PDr tothe well interconnect CPWELL. Subsequently, the memory controller 20outputs a status read command “70h” to the semiconductor memory device10. Upon receipt of the status read command “70h”, the semiconductormemory device 10 outputs to the memory controller 20 data “PASS”indicating that an erasure target block has passed the erase operation.

Next, in the phase P5 for instructing the write operation, the memorycontroller 20 sequentially outputs to the semiconductor memory device 10a write setup command “80h”, addresses “ADD1 to ADD5” of memory cellswhich are a target of a write, write data “DI”, and a write executioncommand “10h”. Upon receipt of the write execution command “10h”, thesequencer 16 of the semiconductor memory device 10 executes the writeoperation according to the write data “DI” on memory cells which are atarget of a write and are designated by the addresses “ADD1 to ADD5”.During the execution of this write operation, the sequencer 16 causesthe ready/busy signal R/Bn to transition from the ready state to thebusy state (R/Bn=“L”). The phase P5 is a write operation with respect toa specific address in a block. In order to perform the write operationwith respect to a part or all of the addresses in the block, the memorycontroller 20 may repeatedly perform the phase P5.

Next, in the phase P3A for setting the judgment level AR3, the memorycontroller 20 sequentially outputs to the semiconductor memory device 10a shift read command “X2h”, an address “01h” indicative of the readvoltage AR, and data “SHIFT” indicative of a shift value from the readvoltage AR. In this manner, the memory controller 20 sets to thesemiconductor memory device 10 the judgment level AR3 used for the shiftread in the lower page read.

Next, in the phase P4A for instructing the lower page read, the memorycontroller 20 sequentially outputs to the semiconductor memory device 10a command “01h” indicative of a lower page, a read setup command “00h”,addresses “ADD1 to ADD5” of measurement target memory cells, and a readexecution command “30h”. Upon receipt of the read execution command“30h”, the sequencer 16 of the semiconductor memory device 10 executesthe read operation using the judgment level AR3 and the read operationat the read voltage ER of the state “E” on measurement target memorycells, which are designated by the addresses “ADD1 to ADD5”. During theexecution of this read operation, the sequencer 16 causes the ready/busysignal R/Bn to transition from the ready state to the busy state(R/Bn=“L”). Thereafter, the sequencer 16 outputs to the memorycontroller 20 the lower page data RLP before the error correction,obtained through the read operation using the judgment level AR3 and theread voltage ER. The memory controller 20 causes the ECC circuit 24 toperform the error correction on the received lower page data RLP beforebeing subjected to the error correction, thereby calculating the lowerpage data CLP after being subjected to the error correction.

Next, in the phase P6 for instructing the one-level read, the memorycontroller 20 sequentially outputs to the semiconductor memory device 10the one-level read command “X1h”, the address “03h” indicative of theread voltage CR of the state “C”, the read setup command “00h”, themeasurement target addresses “ADD1 to ADD5”, and the read executioncommand “30h”. Upon receipt of the read execution command “30h”, thesequencer 16 of the semiconductor memory device 10 executes the readoperation at the read voltage CR on measurement target memory cells,which are designated by the addresses “ADD1 to ADD5”. During theexecution of this read operation, the sequencer 16 causes the ready/busysignal R/Bn to transition from the ready state to the busy state(R/Bn=“L”). Thereafter, the semiconductor memory device 10 outputs tothe memory controller 20 the read result RCR obtained through the readoperation using the read voltage CR.

Thereafter, as described above, the memory controller 20 performs an ANDoperation on the data RLP2 obtained by performing a NOT operation on thelower page data RLP before being subjected to the error correction, andtwo data pieces CLP and RCR, thereby counting the number of OFF bits todetermine the degree of erasure.

In the second example, through the read operation after the writeoperation, the memory controller 20 counts the number of OFF bits amongmeasurement target memory cells in the erasure target block BLKr. Thememory controller 20, based on the number of OFF bits, updates the pulsetime PDr of the erase voltage VERA or maintains without updating thepulse time PDr of the erase voltage VERA.

1.3 Effects of First Embodiment

According to the first embodiment, erasure with respect to memory cellsthrough the erase operation can be optimized by adjusting or updatingthe pulse time of the erase voltage VERA after the erase operation orthe write operation. In other words, by adjusting or updating the pulsetime of the erase voltage VERA, memory cells can be prevented fromtransitioning to the insufficient erasure state or excessive erasurestate through the erase operation.

By preventing memory cells from transitioning to the excessive erasurestate through the erase operation, damage caused to the memory cellsthrough the erase operation can be reduced. Furthermore, by preventingmemory cells from transitioning to the insufficient erasure statethrough the erase operation, for example, read errors can be reduced inpage reads including a read of the state A.

Hereinafter, advantageous effects of the first embodiment will bedescribed in detail by referring to a comparative example. FIG. 21 is adiagram showing a relationship between the number of times a write/eraseis performed on memory cells and the degree of erasure in the memorycells by the erase operation. The increased number of times thewrite/erase is performed encourages the wearing out of memory cells.Thus, as the number of times the write/erase is performed increases,even if the erase operation is performed on memory cells, the amount ofdrawn out electrons stored in a charge storage layer of each memory cellis gradually reduced and the degree of erasure is gradually decreased.Therefore, as shown in FIG. 21, when the erase operation is performedwhile the number of times the erase loop is repeated is set to a fixednumber, the degree of erasure in memory cells is gradually decreased toapproach the upper limit of an acceptable range for the degree oferasure. Accordingly, the number of times the erase loop is repeated isincreased before the degree of erasure exceeds its allowable range. Ifthe number of times the erase loop is repeated is increased, the degreeof erasure in memory cells increases immediately after the increasednumber of times. This causes the wearing out of memory cells.

In the first embodiment described above, based on an erase result (or anerasure state or the degree of erasure) in memory cells which are atarget of erasure after the erase operation or the write operation, thepulse time of the erase voltage VERA used for a next erase operation onthe erasure target memory cells is adjusted or updated.

More specifically, the determination is made from a threshold voltagedistribution state of memory cells in the state “Er” after the eraseoperation or the write operation, as to whether the degree of erasure inthe memory cells corresponds to the insufficient erasure state, theappropriate erasure state, or the excessive erasure state. Based on astate corresponding to the degree of erasure, the pulse time of theerase voltage VERA is adjusted or updated. For example, if the degree oferasure in memory cells corresponds to the insufficient erasure state,the pulse time of the erase voltage VERA is extended. On the other hand,if the degree of erasure in memory cells corresponds to the excessiveerasure state, the pulse time of the erase voltage VERA is shortened.

FIG. 22 shows a relationship between the number of times the write/eraseis performed on memory cells and the degree of erasure by the eraseoperation according to the first embodiment. As described above, in thefirst embodiment, by adjusting the pulse time of the erase voltage VERA,memory cells can be erased delicately, and as shown in FIG. 22, memorycells can be prevented from increasing and decreasing in degree oferasure as compared to the comparative example in FIG. 21. By preventingmemory cells from increasing in degree of erasure, in other words,preventing memory cells from transitioning to the excessive erasurestate, damage caused to the memory cells can be reduced. Furthermore, bypreventing memory cells from decreasing in degree of erasure, in otherwords, preventing memory cells from transitioning to the insufficienterasure state, read errors can be reduced in page reads.

As described above, according to the first embodiment, it is possible toprovide a memory system and a semiconductor memory device which arecapable of reducing damage and read errors in memory cells and improvingthe performance of erase operation.

2. Second Embodiment

Hereinafter, an erase operation in the memory system 1 according to asecond embodiment will be described below. A configuration of the memorysystem 1 according to the second embodiment is similar to that of thefirst embodiment. The explanation of the second embodiment will focusmainly on the points that differ from the first embodiment.

2.1 Erase Operation in Second Embodiment

In the erase operation according to the second embodiment, the initialvoltage value of the erase voltage VERA is adjusted (or changed) basedon an erase result of memory cells after an erase operation or based onan erase result of memory cells after a write operation subsequent tothe erase operation. For example, the initial voltage value of the erasevoltage VERA is increased or decreased. In the second embodiment, theinitial voltage value of the erase voltage VERA, which is adjusted inthis second embodiment, is a voltage value of the erase voltage VERA inthe first erase loop.

2.1.1 First Example of Erase Operation

In the first example, after the erase operation, the initial voltagevalue of the erase voltage VERA is updated based on the erase result ofmemory cells belonging to a word line WL and a string unit SU serving asa measurement target in an erasure target block. An example describedherein is a case in which the initial voltage value of the erase voltageVERA is increased based on an erase result of memory cells.

FIG. 23 is a flowchart showing the first example of the erase operationin the memory system 1 according to the second embodiment. FIG. 24 is adiagram showing operations performed between the memory controller 20and the semiconductor memory device 10. FIG. 25 is a diagram showing anexample of a voltage value management table 22B_2 provided in the memory22 of the memory controller 20. The voltage value management table 22B_2manages the initial voltage value of the erase voltage VERA. In thevoltage value management table 22B_2, a block BLKr (where r is aninteger equal to or greater than 0 and equal to or less than m) isassociated with an initial voltage value PAr used for the eraseoperation on this block BLKr. The processing shown in FIG. 23 isinstructed and controlled by the memory controller 20 (or the CPU 21).

As shown in FIGS. 23 and 24, when the erase operation starts, first, thememory controller 20 transmits to the semiconductor memory device 10 aninitial voltage value PAr of the erase voltage VERA corresponding to theerasure target block BLKr in the voltage value management table 22B_2,thereby setting the initial voltage value PAr to the register 15D_2 ofthe semiconductor memory device 10. More specifically, the memorycontroller 20 obtains from the voltage value management table 22B_2 theinitial voltage value PAr of the erase voltage VERA corresponding to theerasure target block BLKr, transmits the obtained initial voltage valuePAr to the semiconductor memory device 10, and causes the register 15D_2to store the initial voltage value PAr (step S31). For example, when anerasure target block is the block BLK0, the memory controller 20transmits an initial voltage value PA0 associated with the block BLK0 tothe semiconductor memory device 10, thereby causing the register 15D_2to store the initial voltage value PA0.

Next, the memory controller 20 instructs the semiconductor memory device10 to perform the erase operation (step S32). The sequencer 16 of thesemiconductor memory device 10 executes the erase operation on theerasure target block BLKr, using the initial voltage value PAr of theerase voltage VERA stored in the register 15D_2.

Next, the memory controller 20 obtains from the semiconductor memorydevice 10 an erase result of the erase operation on memory cells. Basedon the erase result of the memory cells, obtained from the semiconductormemory device 10, the memory controller 20 determines whether to updatethe initial voltage value PAr of the erase voltage VERA (hereinafter,this determination is also described as “determining the initial voltagevalue of the erase voltage VERA”) (step S33). For example, theprocessing of “determining the initial voltage value of the erasevoltage VERA” in step S33 is performed every time the erase operation isexecuted, every time the write operation/erase operation is executed apredetermined number of times, or when the number of times the writeoperation/erase operation is executed reaches a predetermined number.The processing of “determining the initial voltage value of the erasevoltage VERA” in step S33 will be described later in detail.

Next, based on a determination result of “determining the initialvoltage value of the erase voltage VERA” in step S33, the memorycontroller 20 updates or maintains without updating the initial voltagevalue PAr associated with the erasure target block BLKr in the voltagevalue management table 22B_2 of the memory 22 (step S34). The eraseoperation is thus completed.

Next, the processing of “determining the initial voltage value of theerase voltage VERA” in step S33 in the flowchart shown in FIG. 23 willbe described in detail. FIG. 26 is a flowchart showing the processing of“determining the initial voltage value of the erase voltage VERA” instep S33. The processing shown in FIG. 26 is instructed and controlledby the memory controller 20 (or the CPU 21).

The processing from steps S11 to S14 shown in FIG. 26 is similar to theprocessing from steps S11 to S14 shown in FIG. 11A described above.

As shown in step S11 in FIG. 26, the memory controller 20 sets the readvoltage AR of the state “A” as a read voltage, and further sets a shiftvalue used for making a shift from the read voltage AR. Herein, a readvoltage shifted from the read voltage AR by the shift value is definedas a judgment level AR2. The judgment level AR2 is a voltage level usedfor determining the degree of erasure in memory cells after the eraseoperation. The memory controller 20 sets to the semiconductor memorydevice 10 the judgment level AR2 shifted from the read voltage AR by theshift value. The memory controller 20 further designates to thesemiconductor memory device 10 a word line WL and a string unit SUserving as a measurement target in an erasure target block BLKr (stepS11).

The memory controller 20 instructs the semiconductor memory device 10 toperform the “one-level read” of the state “A” (step S12). Upon receiptof the instruction for the “one-level read” of the state “A”, thesequencer 16 of the semiconductor memory device 10 executes the readoperation at the set judgment level AR2 on measurement target memorycells. The sequencer 16 outputs a read result RA2R of the read operationusing the judgment level AR2, from the semiconductor memory device 10 tothe memory controller 20. As an erase result after the erase operation,the memory controller 20 counts the number of OFF bits from the readresult RA2R. The number of OFF bits is stored in the buffer 22A in thememory 22.

Next, based on the read result of the read operation using the judgmentlevel AR2, the memory controller 20 determines the degree of erasure inmemory cells after the erase operation. That is, the memory controller20 determines the degree of erasure in memory cells after the eraseoperation, from the obtained number of OFF bits (step S13). Morespecifically, the memory controller 20 executes the read operation usingthe judgment level AR2 in the case of determining the degree of erasurein memory cells after the erase operation. When the number of times theread operation is performed has reached the number X, the memorycontroller 20 determines whether or not the average number of OFF bitsin the read operation performed the number X times exceeds the referencevalue Y1 (step S14).

If it is determined in step S14 that the average number of OFF bitsexceeds the reference value Y1 (Yes), the memory controller 20 increasesthe initial voltage value PAr of the erase voltage VERA by apredetermined value (step S18). On the other hand, if the average numberof OFF bits does not exceed the reference value Y1 (No), the memorycontroller 20 terminates the processing of “determining the initialvoltage value of the erase voltage VERA”.

A specific example of the determination in S14 as to whether the numberof OFF bits exceeds the reference value Y1 is similar to that of thefirst embodiment. In the case where the average number of OFF bitsstored in t-3, t-2, t-1, and t-0 of the buffer 22A does not exceed thereference value of 30, the memory controller 20 terminates theprocessing without changing the initial voltage value PAr of the erasevoltage VERA. On the other hand, if the average number of OFF bitsstored in t-3, t-2, t-1, and t-O of the buffer 22A exceeds the referencevalue of 30, the memory controller 20 increases the initial voltagevalue PAr of the erase voltage VERA by a predetermined value, and thenterminates the processing.

The following description is about input/output of commands, addresses,and data between the memory controller 20 and the semiconductor memorydevice 10 in the first example of the erase operation according to thesecond embodiment.

FIG. 27 is a diagram showing a command sequence in the first example ofthe erase operation according to the second embodiment. Output ofcommands and addresses from the memory controller 20 to thesemiconductor memory device 10 and input/output of data between thememory controller 20 and the semiconductor memory device 10, which willbe described below, are performed using the I/O signals DQ0 to DQ7. Thecommand sequence shown in FIG. 27 includes an input/output cycle ofcommands, addresses, and data. A command is expressed by a hexagon, anaddress is expressed by a rounded corner square (or an ellipse), and adata input/output cycle is expressed by a square.

As shown in FIG. 27, the command sequence includes: a phase P11corresponding to “setting the initial voltage value of the erase voltageVERA (step S21)”; a phase P2 corresponding to “instructing the eraseoperation (step S22)”; a phase P3 corresponding to “setting the judgmentlevel AR2 (step S11)” for a shift read in the one-level read; and phasesP3 and P4 corresponding to “instructing the one-level read (step S12)”in the shift read. The phases P2, P3 and P4 are similar to theabove-described phases P2, P3 and P4 shown in FIG. 15.

First, in the phase P11 for setting the initial voltage value of theerase voltage VERA, the memory controller 20 sequentially outputs acommand “0Xh”, an address “01h”, and data “PAr” to the semiconductormemory device 10. The command “0Xh” is a command for designating anerase mode. The address “01h” is an address for setting the initialvoltage value of the erase voltage VERA. The data “PAr” is dataindicative of the initial voltage value of the erase voltage VERAcorresponding to an erasure target block BLKr. In this manner, thememory controller 20 designates an erase mode for the semiconductormemory device 10, and sets to the register 15D_2 of the semiconductormemory device 10 the initial voltage value PAr of the erase voltage VERAused in the erase operation on the erasure target block BLKr.

Next, in the phase P2 for instructing the erase operation, the memorycontroller 20 sequentially outputs an erase setup command “60h”, anaddress “ADD” of the erasure target block BLKr, and the erase executioncommand “D0h” to the semiconductor memory device 10. Upon receipt of theerase execution command “D0h”, the sequencer 16 of the semiconductormemory device 10 executes the erase operation on the erasure targetblock BLKr by applying the erase voltage VERA having the initial voltagevalue PAr to the well interconnect CPWELL.

Thereafter, the memory controller 20 outputs a status read command “70h”to the semiconductor memory device 10. Upon receipt of the status readcommand “70h”, the semiconductor memory device 10 outputs to the memorycontroller 20 data indicative of whether an erasure target block haspassed or failed the erase operation. Herein, for example, thesemiconductor memory device 10 outputs to the memory controller 20 data“PASS” indicating that an erasure target block has passed the eraseoperation.

Next, in the phase P3 for setting the judgment level AR2, the memorycontroller 20 sequentially outputs to the semiconductor memory device 10a one-level read command “X1h” of the state “A”, and an address “01h”indicative of the read voltage AR of the state “A”. Furthermore, thememory controller 20 sequentially outputs to the semiconductor memorydevice 10 a shift read command “X2h”, an address “01h”, and data “SHIFT”indicative of a shift value from the read voltage AR. In this manner,the memory controller 20 sets to the semiconductor memory device 10 thejudgment level AR2 used for the shift read in the one-level read.

Next, in the phase P4 for instructing the read, the memory controller 20sequentially outputs to the semiconductor memory device 10 a read setupcommand “00h”, addresses “ADD1 to ADD5” of measurement target memorycells, and a read execution command “30h”. Upon receipt of the readexecution command “30h”, the sequencer 16 of the semiconductor memorydevice 10 executes the read operation using the judgment level AR2 onmeasurement target memory cells, which are designated by the addresses“ADD1 to ADD5”. Thereafter, the sequencer 16 outputs a read result RA2Robtained through the read operation using the judgment level AR2, fromthe semiconductor memory device 10 to the memory controller 20. Thememory controller 20 counts the number “DO2” of OFF bits which remain,according to the read result RA2R, in the OFF state withouttransitioning to the ON state.

As described above, the example in which the initial voltage value PArof the erase voltage VERA is increased was described as the firstexample. However, the initial voltage PAr of the erase voltage VERA maybe increased or decreased based on the erase result of memory cells, aswith the modification of the first example of the first embodiment.

In the first example, through the read operation after the eraseoperation, the semiconductor memory device 10 outputs to the memorycontroller 20 the number of OFF bits among measurement target memorycells in the erasure target block BLKr. Based on the number of OFF bits,the memory controller 20 updates or maintains without updating theinitial voltage value PAr of the erase voltage VERA.

2.1.2 Second Example of Erase Operation

In the second example, after the write operation subsequent to the eraseoperation, the initial voltage value of the erase voltage VERA isupdated based on the erase result of memory cells belonging to a wordline WL and a string unit SU serving as a measurement target in anerasure target block. An example described herein is a case in which theinitial voltage value of the erase voltage VERA is increased based on anerase result of memory cells. In the second example, the write operationis added between the erase operation and the determination as to whetherto update the initial voltage value.

FIG. 28 is a flowchart showing the second example of the erase operationin the memory system 1 according to the second embodiment. Theprocessing shown in FIG. 28 is instructed and controlled by the memorycontroller 20 (or the CPU 21).

As with the first example, the memory controller 20 transmits to thesemiconductor memory device 10 the initial voltage value PAr of theerase voltage VERA corresponding to the erasure target block BLKr in thevoltage value management table 22B_2 and causes the register 15D_2 ofthe semiconductor memory device 10 to store the initial voltage valuePAr (step S31). The memory controller 20 instructs the semiconductormemory device 10 to perform the erase operation (step S32). Upon receiptof the instruction for the erase operation, the sequencer 16 of thesemiconductor memory device 10 executes the erase operation on theerasure target block BLKr.

After execution of the erase operation on the erasure target block BLKr,the memory controller 20 instructs the semiconductor memory device 10 toexecute the write operation (step S35). Upon receipt of the instructionfor the write operation, the sequencer 16 of the semiconductor memorydevice 10 executes the write operation on write target memory cells inthe erasure target block BLKr.

Next, based on the erase state of memory cells after the writeoperation, the memory controller 20 determines whether to update theinitial voltage value PAr of the erase voltage VERA (hereinafter, thisdetermination is also described as “determining the initial voltagevalue of the erase voltage VERA”) (step S33A). For example, as with thefirst example, the processing of “determining the initial voltage valueof the erase voltage VERA” in step S33A is performed every time thewrite operation is executed after the erase operation, or every time thewrite operation/erase operation is executed a predetermined number oftimes, or when the number of times the write operation/erase operationis executed reaches a predetermined number. The processing of“determining the initial voltage value of the erase voltage VERA” instep S33A will be described later in detail.

Next, based on a determination result of “determining the initialvoltage value of the erase voltage VERA” in step S33A, the memorycontroller 20 updates or maintains without updating the initial voltagevalue PAr associated with the erasure target block BLKr in the voltagevalue management table 22B_2 of the memory 22 (step S34). The eraseoperation is thus completed.

Next, the processing of “determining the initial voltage value of theerase voltage VERA” in step S33A in the flowchart shown in FIG. 28 willbe described in detail. FIG. 29 is a flowchart showing the processing of“determining the initial voltage value of the erase voltage VERA” instep S33A. The processing shown in FIG. 29 is instructed and controlledby the memory controller 20 (or the CPU 21).

The processing from steps S21 to S27 shown in FIG. 29 is similar to theprocessing from steps S21 to S27 shown in FIG. 18 described above.

As shown in FIG. 29, the memory controller 20 sets the read voltage ARof the state “A” as a read voltage, and further sets a shift value usedfor making a shift from the read voltage AR. Herein, a read voltageshifted from the read voltage AR by the shift value is defined as ajudgment level AR3. The judgment level AR3 is a voltage level used todetermine the degree of erasure in memory cells after the eraseoperation. The memory controller 20 sets to the semiconductor memorydevice 10 the judgment level AR3 shifted from the read voltage AR by theshift value. The memory controller 20 further designates to thesemiconductor memory device 10 a word line WL and a string unit SUserving as a measurement target in an erasure target block BLKr (stepS21).

Next, the memory controller 20 instructs the semiconductor memory device10 to execute a “lower page read” (step S22). Upon receipt of theinstruction for the “lower page read”, the sequencer 16 of thesemiconductor memory device 10 executes the read operation using the setjudgment level AR3 and read voltage ER on, for example, measurementtarget memory cells, and obtains lower page data RLP before beingsubjected to error correction. The lower page data RLP is dataimmediately after a read using the judgment level AR3 and the readvoltage ER, and is not yet subjected to the error correction.

Next, the memory controller 20 receives from the semiconductor memorydevice 10 the lower page data RLP before being subjected to the errorcorrection. The memory controller 20 causes the ECC circuit 24 tocorrect an error in the lower page data RLP before being subjected tothe error correction, and obtains the lower page data CLP after beingsubjected to the error correction (step S23).

Next, the memory controller 20 sets the read voltage CR of the state “C”to the semiconductor memory device 10. The memory controller 20 furtherdesignates to the semiconductor memory device 10 a word line WL and astring unit SU serving as a measurement target in an erasure targetblock BLKr (step S24).

Next, the memory controller 20 instructs the semiconductor memory device10 to perform the “one-level read” of the state “C” (step S25). Uponreceipt of the instruction for the “one-level read” of the state “C”,the sequencer 16 executes the read operation at the read voltage CR on,for example, measurement target memory cells, obtains a read result RCRof the state “C”, and outputs it to the memory controller 20.

The memory controller 20 performs an AND operation on the data RLP2 andtwo data pieces CLP and RCR, thereby obtaining the number of OFF bits todetermine the degree of erasure. Herein, the data RLP2 is data obtainedby performing a NOT operation on the lower page data RLP before beingsubjected to the error correction, obtained in step S22, and the twodata pieces CLP and RCR are data obtained in steps S23 and S25. Thememory controller 20 stores the obtained number of OFF bits in thebuffer 22A within the memory 22.

Next, based on the erasure state of memory cells after the writeoperation, the memory controller 20 determines the degree of erasure inerasure target memory cells after the write operation. That is, thememory controller 20 determines the degree of erasure in erasure targetmemory cells after the write operation, from the number of OFF bitsstored in the buffer 22A (step S26). More specifically, in the case ofdetermining the degree of erasure after the write operation subsequentto the erase operation, the memory controller 20 executes the processingin steps S22, S23, and S25 on measurement target memory cells. When thenumber of times the processing in steps S22, S23, and S25 is performedhas reached the number X, the memory controller 20 determines whether ornot the average number of OFF bits in the aforementioned processingperformed the number X times exceeds a reference value Y1 (step S27). Ifit is determined that the average number of OFF bits in theaforementioned processing performed the number X times exceeds thereference value Y1 (Yes), the memory controller 20 increases the initialvoltage value PAr of the erase voltage VERA by a predetermined value(step S29). On the other hand, if the average number of OFF bits in theaforementioned processing performed the number X times does not exceedthe reference value Y1 (No), the memory controller 20 terminates theprocessing of “determining the initial voltage value of the erasevoltage VERA”.

A specific example of the determination in step S27 as to whether thenumber of OFF bits exceeds the reference value Y1 is similar to that ofthe first embodiment.

The following description is about input/output of commands, addresses,and data between the memory controller 20 and the semiconductor memorydevice 10 in the second example of the erase operation according to thesecond embodiment.

FIG. 30 is a diagram showing a command sequence in the second example ofthe erase operation according to the second embodiment Output ofcommands and addresses from the memory controller 20 to thesemiconductor memory device 10 and input/output of data between thememory controller 20 and the semiconductor memory device 10, which willbe described below, are performed using the I/O signals DQ0 to DQ7. Thecommand sequence shown in FIG. 30 includes an input/output cycle ofcommands, addresses, and data. A command is expressed by a hexagon, anaddress is expressed by a rounded corner square (or an ellipse), and adata input/output cycle is expressed by a square.

As shown in FIG. 30, the command sequence includes: a phase P11corresponding to “setting the initial voltage value of the erase voltageVERA (step S31)”; a phase P2 corresponding to “instructing the eraseoperation (step S32)”; a phase P5 corresponding to “instructing thewrite operation (step S35)”; a phase P3A corresponding to “setting thejudgment level AR3 (step S21)” for a shift read in the lower page read;a phase P4A corresponding to “instructing the lower page read (stepS22)” in the shift read; and a phases P6 corresponding to “instructingthe one-level read (step S25)”. The phase P11 is similar to theabove-described phase P11 shown in FIG. 27, and the phases P2, P5, P3A,P4A, and P6 are similar to the above-described phases P2, P5, P3A, P4A,and P6 shown in FIG. 20.

First, in the phase P11 for setting the initial voltage value of theerase voltage VERA, the memory controller 20 sequentially outputs acommand “0Xh” for designating the erase mode, an address “00h”, and data“PAr” to the semiconductor memory device 10. In this manner, the memorycontroller 20 designates an erase mode for the semiconductor memorydevice 10, and sets to the register 15D_2 of the semiconductor memorydevice 10 the initial voltage value PAr of the erase voltage VERA usedin the erase operation on the erasure target block BLKr.

Next, in the phase P2 for instructing the erase operation, the memorycontroller 20 sequentially outputs an erase setup command “60h”, anaddress “ADD” of the erasure target block BLKr, and the erase executioncommand “D0h” to the semiconductor memory device 10. Upon receipt of theerase execution command “D0h”, the sequencer 16 of the semiconductormemory device 10 executes the erase operation on the erasure targetblock BLKr by applying the erase voltage VERA having the initial voltagevalue PAr to the well interconnect CPWELL. Subsequently, the memorycontroller 20 outputs a status read command “70h” to the semiconductormemory device 10. Upon receipt of the status read command “70h”, thesemiconductor memory device 10 outputs to the memory controller 20 data“PASS” indicating that an erasure target block has passed the eraseoperation.

Next, in the phase P5 for instructing the write operation, the memorycontroller 20 sequentially outputs to the semiconductor memory device 10a write setup command “80h”, addresses “ADD1 to ADD5” of write targetmemory cells, write data “DI”, and a write execution command “10h”. Uponreceipt of the write execution command “10h”, the sequencer 16 of thesemiconductor memory device 10 executes the write operation according tothe write data “DI” on write target memory cells which are designated bythe addresses “ADD1 to ADD5”. The phase P5 is a write operation withrespect to a specific address in a block. In order to perform the writeoperation with respect to a part or all of the addresses in the block,the memory controller 20 may repeatedly perform the phase P5.

Next, in the phase P3A for setting the judgment level AR3, the memorycontroller 20 sequentially outputs to the semiconductor memory device 10a shift read command “X2h”, an address “01h” indicative of the readvoltage AR, and data “SHIFT” indicative of a shift value from the readvoltage AR. In this manner, the memory controller 20 sets to thesemiconductor memory device 10 the judgment level AR3 used for the shiftread in the lower page read.

Next, in the phase P4A for instructing the lower page read, the memorycontroller 20 sequentially outputs to the semiconductor memory device 10a command “01h” indicative of a lower page, a read setup command “00h”,addresses “ADD1 to ADD5” of measurement target memory cells, and a readexecution command “30h”. Upon receipt of the read execution command“30h”, the sequencer 16 of the semiconductor memory device 10 executesthe read operation using the judgment level AR3 and the read operationat the read voltage ER on measurement target memory cells which aredesignated by the addresses “ADD1 to ADD5”. Thereafter, the sequencer 16outputs to the memory controller 20 the lower page data “RLP” before theerror correction, obtained through the read operation using the judgmentlevel AR3 and the read voltage ER. The memory controller 20 causes theECC circuit 24 to perform the error correction on the received lowerpage data “RLP” before being subjected to the error correction, therebycalculating the lower page data CLP after being subjected to the errorcorrection.

Next, in the phase P6 for instructing the one-level read, the memorycontroller 20 sequentially outputs to the semiconductor memory device 10the one-level read command “X1h”, the address “03h” indicative of theread voltage CR of the state “C”, the read setup command “00h”, themeasurement target addresses “ADD1 to ADD5”, and the read executioncommand “30h”. Upon receipt of the read execution command “30h”, thesequencer 16 of the semiconductor memory device 10 executes the readoperation at the read voltage CR on measurement target memory cellswhich are designated by the addresses “ADD1 to ADD5”. Thereafter, thesemiconductor memory device 10 outputs to the memory controller 20 theread result RCR obtained through the read operation using the readvoltage CR.

Thereafter, as described above, the memory controller 20 performs an ANDoperation on the data RLP2 obtained by performing a NOT operation on thelower page data RLP before being subjected to the error correction, andtwo data pieces CLP and RCR, thereby counting the number of OFF bits todetermine the degree of erasure.

In the second example, through the read operation after the writeoperation, the memory controller 20 counts the number of OFF bits amongmeasurement target memory cells in the erasure target block BLKr. Basedon the number of OFF bits, the memory controller 20 updates the initialvoltage value PAr of the erase voltage VERA or maintains withoutupdating the initial voltage value PAr of the erase voltage VERA.

2.2 Advantageous Effects of Second Embodiment

According to the second embodiment, erasure with respect to memory cellsthrough the erase operation can be optimized by adjusting or updatingthe initial voltage value of the erase voltage VERA in the first eraseloop after the erase operation or the write operation. In other words,by adjusting or updating the initial voltage value of the erase voltageVERA in the first erase loop, memory cells can be prevented fromtransitioning to the insufficient erasure state or excessive erasurestate through the erase operation.

By preventing memory cells from transitioning to the excessive erasurestate through the erase operation, damage caused to the memory cellsthrough the erase operation can be reduced. Furthermore, by preventingmemory cells from transitioning to the insufficient erasure statethrough the erase operation, for example, read errors can be reduced inpage reads including a read of the state A, for example. The rest of theconfiguration and advantageous effects are similar to those of the firstembodiment.

3. Third Embodiment

Hereinafter, the erase operation in the memory system 1 according to athird embodiment will be described. The configuration of the memorysystem 1 according to the third embodiment is similar to that of thefirst embodiment. The explanation of the third embodiment will focusmainly on the points that differ from the first and second embodiments.

3.1 Erase Operation in Third Embodiment

In the erase operation according to the third embodiment, at least oneof the initial voltage value or the pulse time of the erase voltage VERAis adjusted (or changed) based on an erase result of memory cells afteran erase operation or based on an erase result of memory cells after awrite operation subsequent to the erase operation. In the thirdembodiment, the initial voltage value of the erase voltage VERA, whichis adjusted in this third embodiment, is a voltage value of the erasevoltage VERA in the first erase loop.

3.1.1 First Example of Erase Operation

In the first example, after the erase operation, at least one of theinitial voltage value or the pulse time of the erase voltage VERA isupdated based on the erase result of memory cells belonging to a wordline WL and a string unit SU both serving as a measurement target in anerasure target block. In the example described herein, if the initialvoltage value of the erase voltage VERA does not exceed a referencevalue, a determination is made as to whether to update the initialvoltage value of the erase voltage VERA, and if the initial voltagevalue of the erase voltage VERA exceeds a reference value, adetermination is made as to whether to update the pulse time of theerase voltage VERA.

FIG. 31 is a flowchart showing the first example of the erase operationin the memory system 1 according to the third embodiment. FIG. 32 is adiagram showing operations performed between the memory controller 20and the semiconductor memory device 10. FIG. 33 is a diagram showing anexample of a management table 22B_3 for a pulse time and a voltagevalue, provided in the memory 22 of the memory controller 20. Themanagement table 22B_3 manages the pulse time and the initial voltagevalue of the erase voltage VERA. In the management table 22B_3, eachblock BLKr is associated with a pulse time PDr and an initial voltagevalue PAr used for the erase operation on each associated block BLKr.The processing shown in FIG. 31 is instructed and controlled by thememory controller 20 (or the CPU 21).

As shown in FIGS. 31 and 32, when the erase operation starts, first, thememory controller 20 transmits to the semiconductor memory device 10 apulse time PDr of the erase voltage VERA corresponding to the erasuretarget block BLKr in the management table 22B_3, thereby causing theregister 15D_1 of the semiconductor memory device 10 to store the pulsetime PDr (step S41).

Next, the memory controller 20 transmits to the semiconductor memorydevice 10 the initial voltage value PAr of the erase voltage VERAcorresponding to the erasure target block BLKr in the management table22B_3, and causes the register 15D_2 of the semiconductor memory device10 to store the initial voltage value PAr (step S42).

Next, the memory controller 20 instructs the semiconductor memory device10 to perform the erase operation (step S43). Upon receipt of theinstruction for the erase operation, the sequencer 16 of thesemiconductor memory device 10 executes the erase operation on theerasure target block BLKr, using the pulse time PDr and the initialvoltage value PAr stored in registers 15D_1 and 15D_2.

Next, the memory controller 20 determines whether or not the initialvoltage value PAr of the erase voltage VERA used for the erase operationexceeds a reference value Y (step S44). If the initial voltage value PArdoes not exceed the reference value Y (No), the memory controller 20shifts to the processing of “determining the initial voltage value PArof the erase voltage VERA” (step S45). This processing in step S45 issimilar to the processing in step S33 shown in FIG. 23 described above.

Subsequently, based on a determination result of “determining theinitial voltage value of the erase voltage VERA” in step S45, the memorycontroller 20 updates the initial voltage value PAr associated with theerasure target block BLKr, in the management table 22B_3 of the memory22 (step S46).

On the other hand, in the determination in step S44, if the initialvoltage value PAr of the erase voltage VERA exceeds a reference value Y(Yes), the memory controller 20 shifts to the processing of “determiningthe pulse time of the erase voltage VERA” (step S47). This processing instep S47 is similar to the processing in step S3 shown in FIG. 8described above.

Subsequently, based on a determination result of “determining the pulsetime of the erase voltage VERA” in step S47, the memory controller 20updates the pulse time PDr associated with the erasure target block BLKrin the management table 22B_3 of the memory 22 (step S48). The eraseoperation is thus completed.

The following description is about input/output of commands, addresses,and data between the memory controller 20 and the semiconductor memorydevice 10 in the first example of the erase operation according to thethird embodiment.

FIG. 34 is a diagram showing a command sequence in the first example ofthe erase operation according to the third embodiment. The I/O signalsDQ0 to DQ7 are used for output of commands and addresses from the memorycontroller 20 to the semiconductor memory device 10 and for output ofdata between the memory controller 20 and the semiconductor memorydevice 10, which will be described below. The command sequence shown inFIG. 34 includes an input/output cycle of commands, addresses, and data.A command is expressed by a hexagon, an address is expressed by arounded corner square (or an ellipse) and a data input/output cycle isexpressed by a square.

As shown in FIG. 34, the command sequence includes: a phase P1corresponding to “setting the pulse time of the erase voltage VERA (stepS41)”; a phase P11 corresponding to “setting the initial voltage valueof the erase voltage VERA (step S42)”, a phase P2 corresponding to“instructing the erase operation (step S43)”, a phase P3 correspondingto “setting the judgment level AR2 (step S11)”, and a phase P4corresponding to “instructing the one-level read (step S12)”. The phasesP1, P11, P2, P3, and P4 are similar to the above-described phases P1,P11, P2, P3, and P4 shown in FIG. 15 and FIG. 27.

First, in the phase P1 for setting the pulse time of the erase voltageVERA, the memory controller 20 sequentially outputs a command “0Xh”, anaddress “00h”, and data “PDr” to the semiconductor memory device 10. Thecommand “0Xh” is a command for designating an erase mode. The address“00h” is an address for setting the pulse time of the erase voltageVERA. The data “PDr” is data indicative of the pulse time of the erasevoltage VERA corresponding to an erasure target block BLKr. In thismanner, the memory controller 20 designates an erase mode for thesemiconductor memory device 10, and sets to the register 15D_1 of thesemiconductor memory device 10 the pulse time PDr of the erase voltageVERA used in the erase operation on the erasure target block BLKr.

Next, in the phase P11 for setting the initial voltage value of theerase voltage VERA, the memory controller 20 sequentially outputs acommand “0Xh”, an address “01h”, and data “PAr” to the semiconductormemory device 10. The command “0Xh” is a command for designating anerase mode. The address “01h” is an address for setting the initialvoltage value of the erase voltage VERA. The data “PAr” is dataindicative of the initial voltage value of the erase voltage VERAcorresponding to an erasure target block BLKr. In this manner, thememory controller 20 designates an erase mode for the semiconductormemory device 10, and sets to the register 15D_2 of the semiconductormemory device 10 the initial voltage value PAr of the erase voltage VERAused in the erase operation on the erasure target block BLKr.

Next, in the phase P2 for instructing the erase operation, the memorycontroller 20 sequentially outputs an erase setup command “60h”, anaddress “ADD” of the erasure target block BLKr, and the erase executioncommand “D0h” to the semiconductor memory device 10. Upon receipt of theerase execution command “D0h”, the sequencer 16 of the semiconductormemory device 10 executes the erase operation on the erasure targetblock BLKr by applying the erase voltage VERA having the pulse time PDrand the initial voltage value PAr to the well interconnect CPWELL.

Thereafter, the memory controller 20 outputs a status read command “70h”to the semiconductor memory device 10. Upon receipt of the status readcommand “70h”, the semiconductor memory device 10 outputs to the memorycontroller 20 data “PASS” indicating that an erasure target block haspassed the erase operation.

Next, in the phase P3 for setting the judgment level AR2, the memorycontroller 20 sequentially outputs to the semiconductor memory device 10a one-level read command “X1h” of the state “A”, an address “01h”indicative of the read voltage AR of the state “A”, a shift read command“X2h”, an address “01h”, and data “SHIFT” indicative of a shift valuefrom the read voltage AR. In this manner, the memory controller 20 setsto the semiconductor memory device 10 the judgment level AR2 used forthe shift read in the one-level read.

Next, in the phase P4 for instructing the read, the memory controller 20sequentially outputs to the semiconductor memory device 10 a read setupcommand “00h”, addresses “ADD1 to ADD5”, and a read execution command“30h”. Upon receipt of the read execution command “30h”, the sequencer16 of the semiconductor memory device 10 executes the read operationusing the judgment level AR2 on measurement target memory cells whichare designated by the addresses “ADD1 to ADD5”. Thereafter, thesequencer 16 outputs a read result RA2R read through the read operationat the judgment level AR2, from the semiconductor memory device 10 tothe memory controller 20. The memory controller 20 counts the number“DO3” of OFF bits from the read result RA2R.

In the first example, if the initial voltage value of the erase voltageVERA does not exceed a reference value, the memory controller 20determines whether to update the initial voltage value of the erasevoltage VERA, and if the initial voltage value of the erase voltage VERAexceeds a reference value, the memory controller 20 determines whetherto update the pulse time of the erase voltage VERA.

In the first example, the memory controller 20 first determines whetheror not the initial voltage value of the erase voltage VERA exceeds areference value, and if the initial voltage value exceeds the referencevalue, then determines whether to update the pulse time of the erasevoltage VERA. However, the first example is not limited to this. Theinitial voltage value and the pulse time may be switched in such amanner that the memory controller 20 first determines whether or not thepulse time exceeds a reference value, and if the pulse time exceeds thereference value, then determines whether to update the initial voltagevalue of the erase voltage VERA.

3.1.2 Second Example of Erase Operation

In the second example, after the write operation subsequent to the eraseoperation, at least one of the initial voltage value or the pulse timeof the erase voltage VERA is updated based on the erase result of memorycells belonging to a word line WL and a string unit SU serving as ameasurement target in an erasure target block. In the second example,the write operation is added between the erase operation and thedetermination as to whether the initial voltage value of the erasevoltage VERA exceeds a reference value.

FIG. 35 is a flowchart showing the second example of the erase operationin the memory system 1 according to the third embodiment. The processingshown in FIG. 35 is instructed and controlled by the memory controller20 (or the CPU 21).

As with the first example, the memory controller 20 transmits to thesemiconductor memory device 10 the pulse time PDr of the erase voltageVERA corresponding to the erasure target block BLKr in the managementtable 22B_3, thereby setting the pulse time PDr to the register 15D_1 ofthe semiconductor memory device 10 (step S41).

Next, the memory controller 20 transmits to the semiconductor memorydevice 10 the initial voltage value PAr of the erase voltage VERAcorresponding to the erasure target block BLKr in the management table22B_3, and sets the initial voltage value PAr to the register 15D_2 ofthe semiconductor memory device 10 (step S42).

The memory controller 20 instructs the semiconductor memory device 10 toperform the erase operation (step S43) Upon receipt of the instructionfor the erase operation, the sequencer 16 of the semiconductor memorydevice 10 executes the erase operation on the erasure target block BLKr.

After execution of the erase operation on the erasure target block BLKr,the memory controller 20 instructs the semiconductor memory device 10 toexecute the write operation (step S49). Upon receipt of the instructionfor the write operation, the sequencer 16 of the semiconductor memorydevice 10 executes the write operation on write target memory cells inthe erasure target block BLKr.

The subsequent processing in steps S44, S45, S46, and steps S44, S47,and S48 is similar to that of the first example shown in FIG. 31. Thatis, the memory controller 20 determines whether or not the initialvoltage value PAr of the erase voltage VERA that was used for the eraseoperation exceeds a reference value Y (step S44). If the initial voltagevalue PAr does not exceed the reference value Y (No), the memorycontroller 20 determines whether to update the initial voltage value PAr(step S45). Depending on a result of this determination, the memorycontroller 20 updates or maintains without updating the initial voltagePAr of the erase voltage VERA (step S46).

On the other hand, if the initial voltage value PAr exceeds a referencevalue Y (Yes) in step S44, the memory controller 20 determines whetherto update the pulse time PDr of the erase voltage VERA (step S47).Depending on a result of this determination, the memory controller 20updates or maintains without updating the pulse time PDr of the erasevoltage VERA (step S48). The erase operation is thus completed.

The following description is about input/output of commands, addresses,and data between the memory controller 20 and the semiconductor memorydevice 10 in the second example of the erase operation according to thethird embodiment.

FIG. 36 is a diagram showing a command sequence in the second example ofthe erase operation according to the third embodiment. Output ofcommands and addresses from the memory controller 20 to thesemiconductor memory device 10 and input/output of data between thememory controller 20 and the semiconductor memory device 10, which willbe described below, are performed using the I/O signals DQ0 to DQ7. Thecommand sequence shown in FIG. 36 includes an input/output cycle ofcommands, addresses, and data. A command is expressed by a hexagon, anaddress is expressed by a rounded corner square (or an ellipse), and adata input/output cycle is expressed by a square.

As shown in FIG. 36, the command sequence includes: a phase P1corresponding to “setting the pulse time of the erase voltage VERA (stepS41)”; a phase P11 corresponding to “setting the initial voltage valueof the erase voltage VERA (step S42)”; a phase P2 corresponding to“instructing the erase operation (step S43)”; a phase P5 correspondingto “instructing the write operation (step S49)”; a phase P3Acorresponding to “setting the judgment level AR3 (step S21)” for a shiftread in the lower page read; a phase P4A corresponding to “instructingthe lower page read (step S22)” in the shift read; and a phases P6corresponding to “instructing the one-level read (step S25)”. The phasesP1, P11, P2, P5, P3A, P4A, and P6 are similar to the above-describedphases P1, P11, P2, P5, P3A, P4A, and P6 shown in FIGS. 15, 20, and 27.

As with the first example, first, in the phase P1 for setting the pulsetime of the erase voltage VERA, the memory controller 20 sequentiallyoutputs a command “0Xh”, an address “00h”, and data “PDr” indicative ofa pulse time to the semiconductor memory device 10. In this manner, thememory controller 20 sets the pulse time PDr of the erase voltage VERAto the register 15D_1 of the semiconductor memory device 10.

Next, in the phase P11 for setting the initial voltage value of theerase voltage VERA, the memory controller 20 sequentially outputs acommand “0Xh”, an address “01h”, and data “PAr” indicative of an initialvoltage value to the semiconductor memory device 10. In this manner, thememory controller 20 sets the initial voltage value PAr of the erasevoltage VERA to the register 15D_2 of the semiconductor memory device10.

Next, in the phase P2 for instructing the erase operation, the memorycontroller 20 sequentially outputs an erase setup command “60h”, anaddress “ADD” of the erasure target block BLKr, and the erase executioncommand “D0h” to the semiconductor memory device 10. Upon receipt of theerase execution command “D0h”, the sequencer 16 of the semiconductormemory device 10 executes the erase processing with respect to theerasure target block BLKr by applying the erase voltage VERA having thepulse time PDr and the initial voltage value PAr to the wellinterconnect CPWELL.

Thereafter, the memory controller 20 outputs a status read command “70h”to the semiconductor memory device 10. Upon receipt of the status readcommand “70h”, the semiconductor memory device 10 outputs to the memorycontroller 20 data “PASS” indicating that an erasure target block haspassed the erase operation.

Next, in the phase P5 for instructing the write operation, the memorycontroller 20 sequentially outputs to the semiconductor memory device 10a write setup command “80h”, addresses “ADD1 to ADD5” of write targetmemory cells, write data “DI”, and a write execution command “10h”. Uponreceipt of the write execution command “10h”, the sequencer 16 of thesemiconductor memory device 10 executes the write operation according tothe write data “DI” on write target memory cells which are designated bythe addresses “ADD1 to ADD5”. The phase P5 is a write operation withrespect to a specific address in a block. In order to perform the writeoperation with respect to a part or all of the addresses in the block,the memory controller 20 may repeatedly perform the phase P5.

Next, in the phase P3A for setting the judgment level AR3, the memorycontroller 20 sequentially outputs to the semiconductor memory device 10a shift read command “X2h”, an address “01h” indicative of the readvoltage AR, and data “SHIFT” indicative of a shift value from the readvoltage AR. In this manner, the memory controller 20 sets to thesemiconductor memory device 10 the judgment level AR3 used for the shiftread in the lower page read.

Next, in the phase P4A for instructing the lower page read, the memorycontroller 20 sequentially outputs to the semiconductor memory device 10a command “01h” indicative of a lower page, a read setup command “00h”,addresses “ADD1 to ADD5” of measurement target memory cells, and a readexecution command “30h”. Upon receipt of the read execution command“30h”, the sequencer 16 of the semiconductor memory device 10 executesthe read operation using the judgment level AR3 and the read operationat the read voltage ER of the state “E” on measurement target memorycells which are designated by the addresses “ADD1 to ADD5”. During theexecution of this read operation, the sequencer 16 causes the ready/busysignal R/Bn to transition from the ready state to the busy state(R/Bn=“L”). Thereafter, the sequencer 16 outputs to the memorycontroller 20 the lower page data RLP before being subjected to theerror correction, obtained through the read operation using the judgmentlevel AR3 and the read voltage ER. The memory controller 20 causes theECC circuit 24 to perform the error correction on the received lowerpage data RLP before being subjected to the error correction, therebycalculating the lower page data CLP after being subjected to the errorcorrection.

Next, in the phase P6 for instructing the one-level read, the memorycontroller 20 sequentially outputs to the semiconductor memory device 10the one-level read command “X1h”, the address “03h” indicative of theread voltage CR of the state “C”, the read setup command “00h”, themeasurement target addresses “ADD1 to ADD5”, and the read executioncommand “30h”. Upon receipt of the read execution command “30h”, thesequencer 16 of the semiconductor memory device 10 executes the readoperation at the read voltage CR on measurement target memory cellswhich are designated by the addresses “ADD1 to ADD5”. During theexecution of this read operation, the sequencer 16 causes the ready/busysignal R/Bn to transition from the ready state to the busy state(R/Bn=“L”). Thereafter, the semiconductor memory device 10 outputs tothe memory controller 20 the read result RCR obtained through the readoperation using the read voltage CR.

Thereafter, as described above, the memory controller 20 performs an ANDoperation on the data RLP2 obtained by performing a NOT operation on thelower page data RLP before being subjected to the error correction, andtwo data pieces CLP and RCR, thereby counting the number of OFF bits todetermine the degree of erasure.

In the second example, after the write operation subsequent to the eraseoperation, if the initial voltage value of the erase voltage VERA doesnot exceed a reference value, the memory controller 20 determineswhether to update the initial voltage value of the erase voltage VERA,and if the initial voltage value of the erase voltage VERA exceeds areference value, determines whether to update the pulse time of theerase voltage VERA.

In the second example also, the memory controller 20 first determineswhether the initial voltage value of the erase voltage VERA exceeds areference value, and if the initial voltage value exceeds the referencevalue, then determines whether to update the pulse time of the erasevoltage VERA. However, the second example is not limited to this. Theinitial voltage value and the pulse time may be switched in such amanner that the memory controller 20 first determines whether or not thepulse time exceeds a reference value, and if the pulse time exceeds thereference value, then determines whether to update the initial voltagevalue of the erase voltage VERA. 3.2 Advantageous Effect of ThirdEmbodiment According to the third embodiment, erasure with respect tomemory cells through the erase operation can be optimized by adjustingor updating at least one of the initial voltage value or the pulse timeof the erase voltage VERA in the first erase loop. In other words, byadjusting or updating at least one of the initial voltage value or thepulse time of the erase voltage VERA in the first erase loop, memorycells can be prevented from transitioning to the insufficient erasurestate or excessive erasure state through the erase operation.

By preventing memory cells from transitioning to the excessive erasurestate through the erase operation, damage caused to the memory cellsthrough the erase operation can be reduced. Furthermore, by preventingmemory cells from transitioning to the insufficient erasure statethrough the erase operation, for example, read errors can be reduced inpage reads including a read of the state A. The rest of theconfiguration and advantageous effects are similar to those of the firstembodiment.

4. Fourth Embodiment

Hereinafter, an erase operation in the memory system 1 according to afourth embodiment will be described below. A configuration of the memorysystem 1 according to the fourth embodiment is similar to that of thefirst embodiment. The explanation of the fourth embodiment will focusmainly on the points that differ from the first embodiment.

4.1 Erase Operation in Fourth Embodiment

The first example and second example of an erase operation according tothe fourth embodiment will be described. In the first example, thedegree of erasure in memory cells after the erase operation isdetermined within the semiconductor memory device 10, and the memorycontroller 20 is notified of a result of this determination. In thesecond example, the pulse time of the erase voltage VERA is adjustedbased on an erase result obtained using a plurality of judgment levelsin a read operation.

4.1.1 First Example of Erase Operation

In the first example, after the erase operation, the degree of erasurein memory cells belonging to a word line WL and a string unit SU servingas a measurement target in an erasure target block is determined withinthe semiconductor memory device 10, and the memory controller 20 isnotified of a result of this determination. According to the receivedresult of determination, the memory controller 20 updates the pulse timeof the erase voltage VERA.

The following description is about input/output of commands, addresses,and data between the memory controller 20 and the semiconductor memorydevice 10 in the first example of the erase operation according to thefourth embodiment.

FIG. 37 is a diagram showing a command sequence in the first example ofthe erase operation according to the fourth embodiment. Output ofcommands and addresses from the memory controller 20 to thesemiconductor memory device 10 and input/output of data between thememory controller 20 and the semiconductor memory device 10, which willbe described below, are performed using the I/O signals DQ0 to DQ7.Herein, a judgment level prepared in normal erase verify processing isused as a judgment level used for obtaining the number of OFF bits. Thecommand sequence shown in FIG. 37 includes an input/output cycle ofcommands, addresses, and data. A command is expressed by a hexagon, anaddress is expressed by a rounded corner square (or an ellipse), and adata input/output cycle is expressed by a square.

As shown in FIG. 37, the command sequence includes: a phase P1corresponding to “setting the pulse time of the erase voltage VERA”; aphase P21 corresponding to “instructing the erase operation”; and aphase P22 corresponding to “outputting the degree of erasure(determination result)”.

The phase P21 includes a command “Y0h”, which is a command forinstructing the semiconductor memory device 10 to perform the series ofoperations from the erase operation to the counting of the number of OFFbits. The command “Y0h” includes a phase P3 corresponding to “settingthe judgment level AR2 (step S11)”; a phase P4 or P4A corresponding to“instructing the lower page read (step S22)”; and a phase P6corresponding to “instructing the one-level read (step S25)”.Furthermore, the present embodiment includes the processing in which thesemiconductor memory device 10 performs the counting of the number ofOFF bits, which is performed by the memory controller 20 in the otherembodiments. Accordingly, the number of OFF bits indicative of thedegree of erasure in memory cells can be obtained by the memorycontroller 20 designating the phase P21 only, without designating theaforementioned phases P3, P4 (or P4A), and P6, with respect to thesemiconductor memory device 10.

First, in the phase P1 for setting the pulse time of the erase voltageVERA, the memory controller 20 sequentially outputs a command “0Xh”, anaddress “00h”, and data “PDr” to the semiconductor memory device 10. Thecommand “0Xh” is a command for designating an erase mode. The address“00h” is an address for setting the pulse time of the erase voltageVERA. The data “PDr” is data indicative of the pulse time of the erasevoltage VERA corresponding to an erasure target block BLKr. In thismanner, the memory controller 20 designates an erase mode for thesemiconductor memory device 10, and sets the pulse time PDr of the erasevoltage VERA to the register 15D_1 of the semiconductor memory device10.

Next, in the phase P21 for instructing the erase operation, the memorycontroller 20 sequentially outputs to the semiconductor memory device 10an erase command “Y0h” including the counting of the number of OFF bitsin the erase operation, an erase setup command “60h”, an address “ADD”of the erasure target block BLKr, and the erase execution command “D0h”.Upon receipt of the command “Y0h”, the sequencer 16 of the semiconductormemory device 10 executes the phase P3 corresponding to “setting thejudgment level AR2 (step S11)”, the phase P4 or P4A corresponding to“instructing the lower page read (step S22)”, and the phase P6corresponding to “instructing the one-level read (step S25)”.Thereafter, upon receipt of the erase execution command “D0h”, thesequencer 16 of the semiconductor memory device 10 executes the eraseprocessing on the erasure target block BLKr by applying the erasevoltage VERA of the pulse time PDr to the well interconnect CPWELL.

The sequencer 16 further executes the erase verify processing on theerasure target block BLKr, thereby obtaining the number of OFF bits.That is, the sequencer 16 executes the read operation at a judgmentlevel used for the erase verify processing, with respect to memory cellswithin the erasure target block BLKr, thereby obtaining the number ofOFF bits. The sequencer 16 determines from the obtained number of OFFbits, the degree of erasure in memory cells after the erase operation(that is, the insufficient erasure state, the appropriate erasure state,or the excessive erasure state).

Next, the memory controller 20 outputs the status read command “70h” tothe semiconductor memory device 10. Upon receipt of the status readcommand “70h”, the semiconductor memory device 10 outputs to the memorycontroller 20 data “PASS” indicating that an erasure target block haspassed the erase operation.

Next, as shown in the phase P22, the memory controller 20 outputs to thesemiconductor memory device 10 the status read command “7Xh” foroutputting data indicative of the degree of erasure. Upon receipt of thestatus read command “7Xh”, the sequencer 16 of the semiconductor memorydevice 10 outputs the data “DO5” indicative of the degree of erasure tothe memory controller 20.

Thereafter, the memory controller 20 compares the data “DO5” indicativeof the degree of erasure with a preset threshold value, and according toa result of this comparison, updates the pulse time PDr of the erasevoltage VERA. For example, if the data “DO5” indicative of the degree oferasure indicates the excessive erasure state, the memory controller 20shortens the pulse time PDr of the erase voltage VERA by a predeterminedtime length. If the data “DO5” indicative of the degree of erasureindicates the appropriate erasure state, the memory controller 20maintains without updating the pulse time PDr of the erase voltage VERA.If the data “DO5” indicative of the degree of erasure indicates aninsufficient erasure state, the memory controller 20 extends the pulsetime PDr of the erase voltage VERA by a predetermined time length.

In the first example, after the erase operation, the sequencer 16 withinthe semiconductor memory device 10 determines the degree of erasure inmemory cells based on the number of OFF bits obtained through the readoperation on memory cells in the erasure target block BLKr, therebynotifying the memory controller 20 of data indicative of the degree oferasure. The memory controller 20, based on the data indicative of thedegree of erasure, updates the pulse time PDr of the erase voltage VERAor maintains without updating the pulse time PDr of the erase voltageVERA.

In the first example, the memory controller 20 determines based on dataindicative of the degree of erasure whether to update the pulse time ofthe erase voltage VERA; however, instead of this determination, thememory controller 20 may determine based on data indicative of thedegree of erasure whether to update the initial voltage value of theerase voltage VERA, and update the initial voltage value.

4.1.2 Second Example of Erase Operation

An example described in the second example is a case where after theerase operation, the pulse time of the erase voltage VERA is updatedbased on an erase result obtained through the read operation using aplurality of judgment levels.

FIG. 38 is a flowchart showing the second example of the erase operationin the memory system 1 according to the fourth embodiment. The flowchartshowing the second example of this erase operation is similar to theflowchart showing the first example of the erase operation according tothe first embodiment shown in FIG. 8, except for “determining the pulsetime of the erase voltage VERA” (step S3B). Hereinafter, the processingof “determining the pulse time of the erase voltage VERA” in step S3Bshown in FIG. 38 will be described.

FIG. 39 is a flowchart showing the processing of “determining the pulsetime of the erase voltage VERA” in step S3B shown in FIG. 38. Theprocessing shown in FIG. 39 is instructed and controlled by the memorycontroller 20 (or the CPU 21). FIG. 40 is a diagram showing thresholdvoltage distributions of memory cells corresponding to judgment levelsAR1 to AR4 used in the determination in step S3B. FIG. 41 is a diagramshowing a relationship between the number of OFF bits obtained at thejudgment levels AR1 to AR4 and an erasure state. The judgment levels AR1to AR4 have a magnitude relation expressed as AR4<AR3<AR2<AR1. Thedegree of erasure is determined to be appropriate in the case where thetail of threshold voltage distribution of memory cells in the erasurestate is present between the judgment levels AR3 and AR2.

As shown in FIG. 39, first, the memory controller 20 sets the readvoltage AR of the state “A” as a read voltage, and further sets a shiftvalue F3 used for making a shift from the read voltage AR. Herein, aread voltage shifted from the read voltage AR by the shift value F3 isdefined as a judgment level AR3. The judgment level AR3 is a voltagelevel used to determine the degree of erasure in memory cells after theerase operation. The memory controller 20 sets to the semiconductormemory device 10 the judgment level AR3 (see FIG. 40) shifted from theread voltage AR by the shift value F3. The memory controller 20 furtherdesignates to the semiconductor memory device 10 a word line WL and astring unit SU serving as a measurement target in an erasure targetblock BLKr (step S51).

Next, the memory controller 20 instructs the semiconductor memory device10 to perform the “one-level read” (step S52). Upon receipt of theinstruction for the “one-level read”, the sequencer 16 of thesemiconductor memory device 10 executes the read operation at the setjudgment level AR3 on measurement target memory cells. In this readoperation, a memory cell having a threshold voltage higher than thejudgment level AR3 remains in the OFF state, without transitioning tothe ON state. The sequencer 16 outputs a read result RA3R of the readoperation using the judgment level AR3, from the semiconductor memorydevice 10 to the memory controller 20. From the read result RA3R, thememory controller 20 counts the number of memory cells which remain inthe OFF state without transitioning to the ON state (hereinafter,referred to as the first number of OFF bits). The memory controller 20stores the first number of OFF bits in the buffer 22A within the memory22.

Next, based on the read result of the read operation using the judgmentlevel AR3, the memory controller 20 determines the degree of erasure inmemory cells after the erase operation. That is, the memory controller20 determines the degree of erasure in memory cells after the eraseoperation, based on the first number of OFF bits obtained through theread operation using the judgment level AR3. Specifically, the memorycontroller 20 determines whether or not the first number of OFF bitsexceeds a reference value Y3 (step S53).

In step S53, if the first number of OFF bits exceeds the reference valueY3 (Yes), the memory controller 20 sets the read voltage AR of the state“A” as a read, and further sets a shift value F2 used for making a shiftfrom the read voltage AR. Herein, a read voltage shifted from the readvoltage AR by the shift value F2 is defined as a judgment level AR2. Thejudgment level AR2 is a voltage level used for determining the degree oferasure in memory cells after the erase operation. The memory controller20 sets to the semiconductor memory device 10 the judgment level AR2shifted from the read voltage AR by the shift value F2 (see FIG. 40).The memory controller 20 further designates to the semiconductor memorydevice 10 a word line WL and a string unit SU both serving as ameasurement target in an erasure target block BLKr (step S54).

The memory controller 20 instructs the semiconductor memory device 10 toperform the “one-level read” of the state “A” (step S55). Upon receiptof the instruction for the “one-level read” of the state “A”, thesequencer 16 of the semiconductor memory device 10 executes the readoperation at the set judgment level AR2 on measurement target memorycells. In this read operation, a memory cell having a threshold voltagehigher than the judgment level AR2 remains in the OFF state withouttransitioning to the ON state. The sequencer 16 outputs a read resultRA2R of the read operation using the judgment level AR2, from thesemiconductor memory device 10 to the memory controller 20. From theread result RA2R, the memory controller 20 counts the number of memorycells which remain in the OFF state without transitioning to the ONstate (hereinafter, referred to as the second number of OFF bits). Thememory controller 20 stores the second number of OFF bits in the buffer22A within the memory 22.

Next, based on the read result of the read operation using the judgmentlevel AR2, the memory controller 20 determines the degree of erasure inmemory cells. That is, the memory controller 20 determines the degree oferasure in memory cells based on the second number of OFF bits obtainedthrough the read operation using the judgment level AR2. Specifically,the memory controller 20 determines whether or not the second number ofOFF bits exceeds a reference value Y2 (step S56).

In step S56, if the second number of OFF bits does not exceed thereference value Y2 (No), the memory controller 20 determines that thedegree of erasure in memory cells is the appropriate erasure state, andterminates the processing of determining a pulse time.

On the other hand, in step S56, if the second number of OFF bits exceedsthe reference value Y2 (Yes), the memory controller 20 sets the readvoltage AR of the state “A” as a read voltage, and further sets a shiftvalue F1 used for making a shift from the read voltage AR. Herein, aread voltage shifted from the read voltage AR by the shift value F1 isdefined as a judgment level AR1. The judgment level AR1 is a voltagelevel used to determine the degree of erasure in memory cells after theerase operation. The memory controller 20 sets to the semiconductormemory device 10 the judgment level AR1 shifted from the read voltage ARby the shift value F1 (see FIG. 40). The memory controller 20 furtherdesignates to the semiconductor memory device 10 a word line WL and astring unit SU serving as a measurement target in an erasure targetblock BLKr (step S57).

Next, the memory controller 20 instructs the semiconductor memory device10 to perform the “one-level read” (step S58). Upon receipt of theinstruction for the “one-level read”, the sequencer 16 of thesemiconductor memory device 10 executes the read operation at the setjudgment level AR1 on measurement target memory cells. In this readoperation, a memory cell having a threshold voltage higher than thejudgment level AR1 remains to the OFF state without transitioning to theON state. The sequencer 16 outputs a read result RA1R of the readoperation using the judgment level AR1, from the semiconductor memorydevice 10 to the memory controller 20. From the read result RA1R, thememory controller 20 counts the number of OFF bits which remain in theOFF state without transitioning to the ON state (hereinafter, referredto as the third number of OFF bits). The memory controller 20 stores thethird number of OFF bits in the buffer 22A within the memory 22.

Next, based on the read result of the read operation using the judgmentlevel AR1, the memory controller 20 determines the degree of erasure inmemory cells. That is, the memory controller 20 determines the degree oferasure in memory cells on the basis of the third number of OFF bitsobtained through the read operation using the judgment level AR1.Specifically, the memory controller 20 determines whether or not thethird OFF bits exceed the reference value Y1 (step S59).

In step S59, if the third number of OFF bits does not exceed thereference value Y1 (No), the memory controller 20 determines that thedegree of erasure in memory cells is a slightly insufficient erasurestate, and extends the pulse time PDr of the erase voltage VERA by onestep (step S60). Thereafter, the processing of determining the pulsetime is terminated.

On the other hand, in step S59, if the third number of OFF bits exceedsthe reference value Y1 (Yes), the memory controller 20 determines thatthe degree of erasure in memory cells is the insufficient erasure state,and extends the pulse time PDr of the erase voltage VERA by two steps(step S61). Thereafter, the processing of determining the pulse time isterminated.

Furthermore, in step S53, if the first number of OFF bits does notexceed the reference value Y3 (No), the memory controller 20 sets theread voltage AR of the state “A” as a read voltage, and further sets ashift value F4 used for making a shift from the read voltage AR. Herein,a read voltage shifted from the read voltage AR by the shift value F4 isdefined as a judgment level AR4. The judgment level AR4 is a voltagelevel used to determine a degree of erasure in memory cells after theerase operation. The memory controller 20 sets to the semiconductormemory device 10 the judgment level AR4 shifted from the read voltage ARby the shift value F4 (see FIG. 40). The memory controller 20 furtherdesignates to the semiconductor memory device 10 a word line WL and astring unit SU serving as a measurement target in an erasure targetblock BLKr (step S62).

The memory controller 20 instructs the semiconductor memory device 10 toperform the “one-level read” of the state “A” (step S63). Upon receiptof the instruction for the “one-level read” of the state “A”, thesequencer 16 of the semiconductor memory device 10 executes the readoperation at the set judgment level AR4 on measurement target memorycells. In this read operation, a memory cell having a threshold voltagehigher than the judgment level AR4 remains in the OFF state withouttransitioning to the ON state. The sequencer 16 outputs a read resultRA4R of a read operation using the judgment level AR4, from thesemiconductor memory device 10 to the memory controller 20. From theread result RA4R, the memory controller 20 counts the number of memorycells which remain in the OFF state without transitioning to the ONstate (hereinafter, referred to as the fourth number of OFF bits). Thememory controller 20 stores the fourth number of OFF bits in the buffer22A within the memory 22.

Next, the memory controller 20 determines the degree of erasure inmemory cells based on the read result of the read operation using thejudgment level AR4. That is, the memory controller 20 determines thedegree of erasure in memory cells based on the fourth number of OFF bitsobtained through the read operation using the judgment level AR4.Specifically, the memory controller 20 determines whether or not thefourth number of OFF bits exceeds a reference value Y4 (step S64).

In step S64, if the fourth number of OFF bits exceeds the referencevalue Y4 (Yes), the memory controller 20 determines that the degree oferasure in memory cells is a slightly excessive erasure state, andshortens the pulse time PDr of the erase voltage VERA by one step (stepS65). Thereafter, the processing of determining the pulse time isterminated.

On the other hand, in step S64, if the fourth number of OFF bits doesnot exceed the reference value Y4 (No), the memory controller 20determines that the degree of erasure in memory cells is the excessiveerasure state, and shortens the pulse time PDr of the erase voltage VERAby two steps (step S66). Thereafter, the processing of determining thepulse time is terminated.

In the second example, after the erase operation, the memory controller20 determines, based on the number of OFF bits obtained through the readoperation performed using a plurality of judgment levels on measurementtarget memory cells in an erasure target block, whether erasure withrespect to memory cells corresponds to the excessive erasure state, theslightly excessive erasure state, the appropriate erasure state, theslightly insufficient erasure state, or the insufficient erasure state.Based on results of the above determinations, the memory controller 20updates the pulse time PDr of the erase voltage VERA by four-stagesteps, or maintains without updating the pulse time PDr of the erasevoltage VERA.

In the second example, the memory controller 20 determines, based on adetermination result of the degree of erasure, whether to update thepulse time PDr of the erase voltage VERA, and updates the pulse timePDr; however, instead of this determination, the memory controller 20may determine whether to update the initial voltage value PAr of theerase voltage VERA and update the initial voltage value PAr.

4.1.3 Third Example of Erase Operation

A third example corresponds to another aspect of the second example.Described in the third example is another example in which after theerase operation, the pulse time of the erase voltage VERA is updatedbased on an erase result obtained through the read operation using aplurality of judgment levels.

The flowchart showing the third example of the erase operation issimilar to the flowchart showing the second example shown in FIG. 38,except for “determining the pulse time of the erase voltage VERA” (stepS3B).

Hereinafter, the processing of “determining the pulse time of the erasevoltage VERA” which differs from that of the second example will bedescribed with reference to FIG. 42. FIG. 42 is a flowchart showing theprocessing of “determining the pulse time of the erase voltage VERA” inthe third example of the erase operation. The processing shown in FIG.42 is instructed and controlled by the memory controller 20 (or the CPU21).

As shown in FIG. 42, first, the memory controller 20 sets the readvoltage AR of the state “A” as a read voltage, and further sets a shiftvalue F3 used for making a shift from the read voltage AR. Herein, aread voltage shifted from the read voltage AR by the shift value F3 isdefined as a judgment level AR3. The judgment level AR3 is a voltagelevel used to determine the degree of erasure in memory cells after theerase operation. The memory controller 20 sets to the semiconductormemory device 10 the judgment level AR3 shifted from the read voltage ARby the shift value F3. The memory controller 20 further designates tothe semiconductor memory device 10 a word line WL and a string unit SUserving as a measurement target in an erasure target block BLKr (stepS71).

Next, the memory controller 20 instructs the semiconductor memory device10 to perform the “one-level read” of the state “A” (step S72). Uponreceipt of the instruction for the “one-level read” of the state “A”,the sequencer 16 of the semiconductor memory device 10 executes the readoperation at the set judgment level AR3 on measurement target memorycells. The sequencer 16 outputs a read result RA3R of the read operationusing the judgment level AR3, from the semiconductor memory device 10 tothe memory controller 20. The memory controller 20 counts the number ofmemory cells in the OFF state (the first number of OFF bits) from theread result RA3R.

Next, the memory controller 20 sets the read voltage AR of the state “A”as a read voltage, and further sets a shift value F4 used for making ashift from the read voltage AR. Herein, a read voltage shifted from theread voltage AR by the shift value F4 is defined as a judgment levelAR4. The judgment level AR4 is a voltage level used to determine adegree of erasure in memory cells after the erase operation. The memorycontroller 20 sets to the semiconductor memory device 10 the judgmentlevel AR4 shifted from the read voltage AR by the shift value F4. Thememory controller 20 further designates to the semiconductor memorydevice 10 a word line WL and a string unit SU serving as a measurementtarget in an erasure target block BLKr (step S73).

The memory controller 20 instructs the semiconductor memory device 10 toperform the “one-level read” of the state “A” (step S74). Upon receiptof the instruction for the “one-level read” of the state “A”, thesequencer 16 executes the read operation at the set judgment level AR4on measurement target memory cells. The sequencer 16 outputs a readresult RA4R of the read operation using the judgment level AR4, from thesemiconductor memory device 10 to the memory controller 20. The memorycontroller 20 counts the number of memory cells in the OFF state (thesecond number of OFF bits) from the read result RA4R.

Next, the memory controller 20 sets the read voltage AR of the state “A”as a read voltage, and further sets a shift value F2 used for making ashift from the read voltage AR. Herein, a read voltage shifted from theread voltage AR by the shift value F2 is defined as a judgment levelAR2. The judgment level AR2 is a voltage level used for determining thedegree of erasure in memory cells after the erase operation. The memorycontroller 20 sets to the semiconductor memory device 10 the judgmentlevel AR2 shifted from the read voltage AR by the shift value F2. Thememory controller 20 further designates to the semiconductor memorydevice 10 a word line WL and a string unit SU both serving as ameasurement target in an erasure target block BLKr (step S75).

The memory controller 20 then instructs the semiconductor memory device10 to perform the “one-level read” of the state “A” (step S76). Uponreceipt of the instruction for the “one-level read” of the state “A”,the sequencer 16 executes the read operation at the set judgment levelAR2 on measurement target memory cells. The sequencer 16 outputs a readresult RA2R of the read operation using the judgment level AR2, from thesemiconductor memory device 10 to the memory controller 20. The memorycontroller 20 counts the number of memory cells in the OFF state (thethird number of OFF bits) from the read result RA2R.

Next, the memory controller 20 sets the read voltage AR of the state “A”as a read voltage, and further sets a shift value F1 used for making ashift from the read voltage AR. Herein, a read voltage shifted from theread voltage AR by the shift value F1 is defined as a judgment levelAR1. The judgment level AR1 is a voltage level used to determine adegree of erasure in memory cells after the erase operation. The memorycontroller 20 sets to the semiconductor memory device 10 the judgmentlevel AR1 shifted from the read voltage AR by the shift value F1. Thememory controller 20 further designates to the semiconductor memorydevice 10 a word line WL and a string unit SU serving as a measurementtarget in an erasure target block BLKr (step S77).

Next, the memory controller 20 instructs the semiconductor memory device10 to perform the “one-level read” of the state “A” (step S78). Uponreceipt of the instruction for the “one-level read” of the state “A”,the sequencer 16 executes the read operation at the set judgment levelAR1 on measurement target memory cells. The sequencer 16 outputs a readresult RA1R of the read operation using the judgment level AR1, from thesemiconductor memory device 10 to the memory controller 20. The memorycontroller 20 counts the number of memory cells in the OFF state (thefourth number of OFF bits) from the read result RA1R.

Next, the memory controller 20 determines the degree of erasure inmemory cells depending on whether or not the first number to the fourthnumber of OFF bits obtained through the read operations at the readvoltages AR1 to AR4 exceed the reference value. The memory controller 20further updates the pulse time PDr based on the determination result ofthe degree of erasure (step S79). The processing of determining thepulse time is thus terminated.

As with the second example, in the third example, the memory controller20 determines, based on the number of OFF bits obtained through the readoperation performed using a plurality of judgment levels, whethererasure with respect to memory cells corresponds to the excessiveerasure state, the slightly excessive erasure state, the appropriateerasure state, the slightly insufficient erasure state, or theinsufficient erasure state. Furthermore, based on results of the abovedeterminations, the memory controller 20 updates or maintains withoutupdating the pulse time PDr of the erase voltage VERA.

In the third example, the memory controller 20 determines based on thedetermination result of the degree of erasure, whether to update thepulse time PDr of the erase voltage VERA, and then updates the pulsetime PDr; however, instead of this determination, the memory controller20 may determine whether to update the initial voltage value PAr of theerase voltage VERA and update the initial voltage value PAr.

4.2 Advantageous Effects of Fourth Embodiment

According to the fourth embodiment, erasure with respect to memory cellsthrough the erase operation can be optimized by adjusting or updatingthe pulse time of the erase voltage VERA after the erase operation. Inother words, by adjusting or updating the pulse time of the erasevoltage VERA, memory cells can be prevented from transitioning to theinsufficient erasure state or excessive erasure state through the eraseoperation.

By preventing memory cells from transitioning to the excessive erasurestate through the erase operation, damage caused to the memory cellsthrough the erase operation can be reduced. Furthermore, by preventingmemory cells from transitioning to the insufficient erasure statethrough the erase operation, for example, read errors can be reduced inpage reads including a read of the state A. The rest of theconfiguration and advantageous effects are similar to those of the firstembodiment.

5. Other Modifications, Etc

Moreover, in the above-described embodiment, a NAND flash memory wasdescribed as an example of a semiconductor memory device; however, theembodiment is not limited to a NAND flash memory, and is applicable toother semiconductor memories in general. Furthermore, the presentembodiment is applicable to various memory devices other than asemiconductor memory. Furthermore, the order of the steps in theabove-described flowchart may be altered in any manner possible.

The embodiments described above are presented merely as examples and arenot intended to restrict the scope of the invention. These embodimentsmay be implemented in various other forms, and various omissions,replacements, and changes can be made without departing from the gist ofthe invention. The embodiments and their modifications are included inthe scope and spirit of the invention and are included in the scope ofthe claimed inventions and their equivalents.

What is claimed is:
 1. A memory system comprising: a semiconductormemory device including a first memory cell configured to store data;and a controller configured to output a first parameter and a firstcommand, the first parameter relating to an erase voltage for a firsterase operation with respect to the first memory cell, the first commandinstructing the first erase operation wherein the controller isconfigured to output the first command after outputting the firstparameter to the semiconductor memory device, the first erase operationincludes a first processing that contains erase processing and eraseverify processing, the erase processing applying an erase voltage to thefirst memory cell, the erase verify processing verifying as to whether aresult of verify processing executed with respect to the first memorycell is pass or fail, the controller is configured to execute the firstprocessing again when the result is fail, and terminate the first eraseoperation when the result is pass, the semiconductor memory deviceincludes a plurality of memory cells including the first memory cell,and after the first erase operation is executed with respect to theplurality of memory cells and then a write operation is executed withrespect to a portion of the plurality of memory cells, the controller isconfigured to update the first parameter based on a result of a readoperation with respect to at least another portion of the plurality ofmemory cells after the first erase operation.
 2. The memory systemaccording to claim 1, wherein the first parameter includes at least oneof a pulse time and a voltage value of an erase voltage that is appliedto the first memory cell when the first erase operation is executed. 3.The memory system according to claim 1, wherein: the semiconductormemory device includes a block containing the plurality of memory cellsincluding the first memory cell; and the controller is configured toinstruct the first erase operation with respect to the block.
 4. Thememory system according to claim 1, wherein: the semiconductor memorydevice includes the plurality of memory cells including the first memorycell; and the controller is configured to: output a second parameterrelating to a first voltage for a read operation with respect to thefirst memory cell; obtain a first number that is a number of memorycells in an OFF state among the plurality of memory cells during theread operation; and update the first parameter when the first number isgreater than a first value.
 5. The memory system according to claim 1,wherein: the semiconductor memory device is configured to output to thecontroller a result of a read operation with respect to the first memorycell after the first erase operation; and the controller is configuredto update the parameter based on the result.
 6. The memory systemaccording to claim 1, wherein the controller is configured to store atable including the parameter with respect to the first memory cell. 7.The memory system according to claim 1, wherein the semiconductor memorydevice is configured to execute the erase processing using the firstparameter.
 8. The memory system according to claim 1, wherein if thefirst erase operation with respect to the first memory cell is executeda plurality of times, every time the first erase operation is executed,the controller is configured to update the first parameter based on aresult of a read operation with respect to the first memory cell afterthe first erase operation.
 9. The memory system according to claim 1,wherein if the first erase operation with respect to the first memorycell is executed a plurality of times, every time the first eraseoperation is executed a predetermined number of times, the controller isconfigured to update the first parameter based on a result of a readoperation with respect to the first memory cell after the first eraseoperation.
 10. The memory system according to claim 4, wherein thesemiconductor memory device includes a first word line connected to theplurality of memory cells.
 11. The memory system according to claim 1,wherein: the semiconductor memory device includes the plurality ofmemory cells including the first memory cell, and includes a pluralityof first word lines connected to the plurality of memory cells; and thecontroller is configured to: output a second parameter relating to afirst voltage for a read operation with respect to the first memorycell; obtain a second number that is a number of memory cells in an OFFstate among the plurality of memory cells during the read operation perfirst word line of the first word lines; and update the first parameterwhen the second number is greater than a first value.
 12. The memorysystem according to claim 1, wherein: the semiconductor memory deviceincludes the plurality of memory cells including the first memory cell,and further includes a second word line connected to a portion of theplurality of memory cells and a third word line connected to anotherportion of the plurality of memory cells; and the controller isconfigured to: output a second parameter relating to a first voltage fora read operation with respect to the first memory cell; obtain a thirdnumber that is a number of memory cells in an OFF state among theplurality of memory cells during the read operation on the second wordline; obtain a fourth number that is a number of memory cells in an OFFstate among the plurality of memory cells during the read operation onthe third word line; and update the first parameter when at least one ofthe third number and the fourth number is greater than a first value.13. A memory system comprising: a semiconductor memory device includinga first memory cell configured to store data; and a controllerconfigured to output a first parameter and a first command, the firstparameter relating to an erase voltage for a first erase operation withrespect to the first memory cell, the first command instructing thefirst erase operation, wherein the controller is configured to outputthe first command after outputting the first parameter to thesemiconductor memory device, the semiconductor memory device includes aplurality of memory cells including the first memory cell; thecontroller is configured to: output a second parameter relating to afirst voltage for a first read operation with respect to the firstmemory cell, and obtain a first number that is a number of memory cellsin an OFF state among the plurality of memory cells during the firstread operation; when the first number is greater than a first value,output a third parameter relating to a second voltage for a second readoperation with respect to the first memory cell, obtain a second numberthat is a number of memory cells in an OFF state among the plurality ofmemory cells during the second read operation, and update the firstparameter based on whether or not the second number is greater than asecond value; and when the first number is smaller than the first value,output a fourth parameter relating to a third voltage for a third readoperation with respect to the first memory cell, obtain a third numberthat is a number of memory cells in an OFF state among the plurality ofmemory cells during the third read operation, and update the firstparameter based on whether or not the third number is greater than athird value; the second voltage is greater than the first voltage; andthe third voltage is smaller than the first voltage.
 14. The memorysystem according to claim 1, wherein: the semiconductor memory deviceincludes the plurality of memory cells including the first memory cell;and the plurality of memory cells are stacked above a semiconductorsubstrate.
 15. The memory system according to claim 4, wherein if thefirst erase operation with respect to the plurality of memory cells isexecuted a plurality of times, the controller is configured to changethe first value in accordance with a number of times the first eraseoperation is executed.
 16. The memory system according to claim 1,wherein: after outputting the first command to the semiconductor memorydevice, the controller is configured to output a second command afteroutputting a second parameter different from the first parameter to thesemiconductor memory device, the second parameter relates to an erasevoltage for a second erase operation with respect to the first memorycell, and the second command instructs the second erase operation.
 17. Asemiconductor memory device comprising: a plurality of memory cellsconfigured to store data; and a control circuit configured to execute anerase operation with respect to the plurality of memory cells, whereinthe control circuit is configured to receive a parameter relating to anerase voltage for the erase operation, then receive a first commandinstructing the erase operation, and thereafter execute the eraseoperation using the parameter, and after the erase operation is executedwith respect to the plurality of memory cells and then a write operationis executed with respect to a portion of the plurality of memory cells,the parameter is updated based on a result of a read operation withrespect to at least another portion of the plurality of memory cellsafter the erase operation.
 18. The semiconductor memory device accordingto claim 17, wherein if the control circuit receives a second commanddesignating that a use target of the parameter is the erase operation,then receives an address designating a type of the parameter, thenreceives data indicating a set value of the parameter, then receives thefirst command, then receives an address of a target of the eraseoperation, and then receives a third command for starting the eraseoperation, the control circuit is configured to execute the eraseoperation with respect to the plurality of memory cells that is thetarget of the erase operation.